@misc{PurintonBookhagen2017, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Validation of digital elevation models (DEMs) and comparison of geomorphic metrics on the southern Central Andean Plateau}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-396277}, pages = {27}, year = {2017}, abstract = {In this study, we validate and compare elevation accuracy and geomorphic metrics of satellite-derived digital elevation models (DEMs) on the southern Central Andean Plateau. The plateau has an average elevation of 3.7 km and is characterized by diverse topography and relief, lack of vegetation, and clear skies that create ideal conditions for remote sensing. At 30m resolution, SRTM-C, ASTER GDEM2, stacked ASTER L1A stereopair DEM, ALOS World 3D, and TanDEM-X have been analyzed. The higher-resolution datasets include 12m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X DEMs, and 5m ALOS World 3D. These DEMs are state of the art for optical (ASTER and ALOS) and radar (SRTM-C and TanDEM-X) spaceborne sensors. We assessed vertical accuracy by comparing standard deviations of the DEM elevation versus 307 509 differential GPS measurements across 4000m of elevation. For the 30m DEMs, the ASTER datasets had the highest vertical standard deviation at > 6.5 m, whereas the SRTM-C, ALOS World 3D, and TanDEM-X were all < 3.5 m. Higher-resolution DEMs generally had lower uncertainty, with both the 12m TanDEM-X and 5m ALOSWorld 3D having < 2m vertical standard deviation. Analysis of vertical uncertainty with respect to terrain elevation, slope, and aspect revealed the low uncertainty across these attributes for SRTM-C (30 m), TanDEM-X (12-30 m), and ALOS World 3D (5-30 m). Single-CoSSC TerraSAR-X/TanDEM-X 10m DEMs and the 30m ASTER GDEM2 displayed slight aspect biases, which were removed in their stacked counterparts (TanDEM-X and ASTER Stack). Based on low vertical standard deviations and visual inspection alongside optical satellite data, we selected the 30m SRTM-C, 12-30m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X, and 5m ALOS World 3D for geomorphic metric comparison in a 66 km2 catchment with a distinct river knickpoint. Consistent m=n values were found using chi plot channel profile analysis, regardless of DEM type and spatial resolution. Slope, curvature, and drainage area were calculated and plotting schemes were used to assess basin-wide differences in the hillslope-to-valley transition related to the knickpoint. While slope and hillslope length measurements vary little between datasets, curvature displays higher magnitude measurements with fining resolution. This is especially true for the optical 5m ALOS World 3D DEM, which demonstrated high-frequency noise in 2-8 pixel steps through a Fourier frequency analysis. The improvements in accurate space-radar DEMs (e.g., TanDEM-X) for geomorphometry are promising, but airborne or terrestrial data are still necessary for meter-scale analysis.}, language = {en} } @misc{AtmaniBookhagenSmith2022, author = {Atmani, Farid and Bookhagen, Bodo and Smith, Taylor}, title = {Measuring Vegetation Heights and Their Seasonal Changes in the Western Namibian Savanna Using Spaceborne Lidars}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1275}, issn = {1866-8372}, doi = {10.25932/publishup-56991}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-569915}, pages = {20}, year = {2022}, abstract = {The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) with its land and vegetation height data product (ATL08), and Global Ecosystem Dynamics Investigation (GEDI) with its terrain elevation and height metrics data product (GEDI Level 2A) missions have great potential to globally map ground and canopy heights. Canopy height is a key factor in estimating above-ground biomass and its seasonal changes; these satellite missions can also improve estimated above-ground carbon stocks. This study presents a novel Sparse Vegetation Detection Algorithm (SVDA) which uses ICESat-2 (ATL03, geolocated photons) data to map tree and vegetation heights in a sparsely vegetated savanna ecosystem. The SVDA consists of three main steps: First, noise photons are filtered using the signal confidence flag from ATL03 data and local point statistics. Second, we classify ground photons based on photon height percentiles. Third, tree and grass photons are classified based on the number of neighbors. We validated tree heights with field measurements (n = 55), finding a root-mean-square error (RMSE) of 1.82 m using SVDA, GEDI Level 2A (Geolocated Elevation and Height Metrics product): 1.33 m, and ATL08: 5.59 m. Our results indicate that the SVDA is effective in identifying canopy photons in savanna ecosystems, where ATL08 performs poorly. We further identify seasonal vegetation height changes with an emphasis on vegetation below 3 m; widespread height changes in this class from two wet-dry cycles show maximum seasonal changes of 1 m, possibly related to seasonal grass-height differences. Our study shows the difficulties of vegetation measurements in savanna ecosystems but provides the first estimates of seasonal biomass changes.}, language = {en} } @article{PurintonBookhagen2020, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Multiband (X, C, L) radar amplitude analysis for a mixed sand- and gravel-bed river in the eastern Central Andes}, series = {Remote sensing of environment : an interdisciplinary journal}, volume = {246}, journal = {Remote sensing of environment : an interdisciplinary journal}, publisher = {Elsevier}, address = {New York}, issn = {0034-4257}, doi = {10.1016/j.rse.2020.111799}, pages = {16}, year = {2020}, abstract = {Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These backscatter signals are often exploited to assess the roughness of plowed agricultural fields and water surfaces, and less so to complex, heterogeneous geological surfaces. The bedload of mixed sand- and gravel-bed rivers can be considered a mixture of smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river in the eastern Central Andes with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we present theory and hypotheses regarding radar response to an alluvial channel bed. We test our hypotheses by comparing backscatter responses over vegetation-free endmember surfaces from inside and outside of the active channel-bed area. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the local variability of sand-dominated patches is analyzed using Fourier frequency analysis, by fitting stretched-exponential and power-law regression models to the 2-D power spectrum of backscatter amplitude. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR amplitude signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength L-band radar, whereas C- and X-band is sensitive only to sand variability. Because the spatial extent of smooth sand patches in our study area is typically< 50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity in alluvial rivers with aerially exposed bedload using SAR amplitude.}, language = {en} } @article{PurintonBookhagen2021, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Tracking downstream variability in large grain-size distributions in the South-Central Andes}, series = {Journal of geophysical research : F, Earth surface}, volume = {126}, journal = {Journal of geophysical research : F, Earth surface}, number = {8}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1029/2021JF006260}, pages = {1 -- 29}, year = {2021}, abstract = {Mixed sand- and gravel-bed rivers record erosion, transport, and fining signals in their bedload size distributions. Thus, grain-size data are imperative for studying these processes. However, collecting hundreds to thousands of pebble measurements in steep and dynamic high-mountain river settings remains challenging. Using the recently published digital grain-sizing algorithm PebbleCounts, we were able to survey seven large (>= 1,000 m2) channel cross-sections and measure thousands to tens-of-thousands of grains per survey along a 100-km stretch of the trunk stream of the Toro Basin in Northwest Argentina. The study region traverses a steep topographic and environmental gradient on the eastern margin of the Central Andean Plateau. Careful counting and validation allows us to identify measurement errors and constrain percentile uncertainties using large sample sizes. In the coarse >= 2.5 cm fraction of bedload, only the uppermost size percentiles (>= 95th) vary significantly downstream, whereas the 50th and 84th percentiles show less variability. We note a relation between increases in these upper percentiles and along-channel junctions with large, steep tributaries. This signal is strongly influenced by lithology and geologic structures, and mixed with local hillslope input. In steep catchments like the Toro Basin, we suggest nonlinear relationships between geomorphic metrics and grain size, whereby the steepest parts of the landscape exert primary control on the upper grain-size percentiles. Thus, average or median metrics that do not apply weights or thresholds to steeper topography may be less predictive of grain-size distributions in such settings.}, language = {en} } @article{OlenBookhagen2020, author = {Olen, Stephanie M. and Bookhagen, Bodo}, title = {Applications of SAR interferometric coherence time series}, series = {Journal of geophysical research : Earth surface}, volume = {125}, journal = {Journal of geophysical research : Earth surface}, number = {3}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1029/2019JF005141}, pages = {22}, year = {2020}, abstract = {Sediment transport domains in mountain landscapes are characterized by fundamentally different processes and rates depending on several factors, including geology, climate, and biota. Accurately identifying where transitions between transport domains occur is an important step to quantify the past, present, and future contribution of varying erosion and sedimentation processes and enhance our predictive capabilities. We propose a new methodology based on time series of synthetic aperture radar (SAR) interferometric coherence images to map sediment transport regimes across arid and semiarid landscapes. Using 4 years of Sentinel-1 data, we analyze sediment transport regimes for the south-central Andes in northwestern Argentina characterized by steep topographic and climatic gradients. We observe seasonally low coherence during the regional wet season, particularly on hillslopes and in alluvial channels. The spatial distribution of coherence is compared to drainage areas extracted from digital topography to identify two distinct transitions within watersheds: (a) a hillslope-to-fluvial and (b) a fluvial-to-alluvial transition. While transitions within a given basin can be well-constrained, the relative role of each sediment transport domain varies widely over the climatic and topographic gradients. In semiarid regions, we observe larger relative contributions from hillslopes compared to arid regions. Across regional gradients, the range of coherence within basins positively correlates to previously published millennial catchment-wide erosion rates and to topographic metrics used to indicate long-term uplift. Our study suggests that a dense time series of interferometric coherence can be used as a proxy for surface sediment movement and landscape stability in vegetation-free settings at event to decadal timescales.}, language = {en} } @article{SmithRheinwaltBookhagen2021, author = {Smith, Taylor and Rheinwalt, Aljoscha and Bookhagen, Bodo}, title = {Topography and climate in the upper Indus Basin}, series = {The science of the total environment : an international journal for scientific research into the environment and its relationship with man}, volume = {786}, journal = {The science of the total environment : an international journal for scientific research into the environment and its relationship with man}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0048-9697}, doi = {10.1016/j.scitotenv.2021.147363}, pages = {11}, year = {2021}, abstract = {The Upper Indus Basin (UIB), which covers a wide range of climatic and topographic settings, provides an ideal venue to explore the relationship between climate and topography. While the distribution of snow and glaciers is spatially and temporally heterogeneous, there exist regions with similar elevation-snow relationships. In this work, we construct elevation-binned snow-cover statistics to analyze 3415 watersheds and 7357 glaciers in the UIB region. We group both glaciers and watersheds using a hierarchical clustering approach and find that (1) watershed clusters mirror large-scale moisture transport patterns and (2) are highly dependent on median watershed elevation. (3) Glacier clusters are spatially heterogeneous and are less strongly controlled by elevation, but rather by local topographic parameters that modify solar insolation. Our clustering approach allows us to clearly define self-similar snow-topographic regions. Eastern watersheds in the UIB show a steep snow cover-elevation relationship whereas watersheds in the central and western UIB have moderately sloped relationships, but cluster in distinct groups. We highlight this snow-cover-topographic transition zone and argue that these watersheds have different hydrologic responses than other regions. Our hierarchical clustering approach provides a potential new framework to use in defining climatic zones in the cyrosphere based on empirical data.}, language = {en} } @article{SmithBookhagen2021, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Climatic and biotic controls on topographic asymmetry at the global scale}, series = {Journal of geophysical research : JGR, Earth surface}, volume = {126}, journal = {Journal of geophysical research : JGR, Earth surface}, number = {1}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1029/2020JF005692}, pages = {24}, year = {2021}, abstract = {Insolation differences play a primary role in controlling microclimate and vegetation cover, which together influence the development of topography. Topographic asymmetry (TA), or slope differences between terrain aspects, has been well documented in small-scale, field-based, and modeling studies. Here we combine a suite of environmental (e.g., vegetation, temperature, solar insolation) and topographic (e.g., elevation, drainage network) data to explore the driving mechanisms and markers of TA on a global scale. Using a novel empirical TA analysis method, we find that (1) steeper terrain has higher TA magnitudes, (2) globally, pole-facing terrain is on average steeper than equator-facing terrain, especially in mid-latitude, tectonically quiescent, and vegetated landscapes, and (3) high-elevation and low-temperature regions tend to have terrain steepened toward the equator. We further show that there are distinct differences in climate and vegetation cover across terrain aspects, and that TA is reflected in the size and form of fluvial drainage networks. Our work supports the argument that insolation asymmetries engender differences in local microclimates and vegetation on opposing terrain aspects, which broadly encourage the development of asymmetric topography across a range of lithologic, tectonic, geomorphic, and climatic settings.}, language = {en} } @article{AtmaniBookhagenSmith2022, author = {Atmani, Farid and Bookhagen, Bodo and Smith, Taylor}, title = {Measuring vegetation heights and their seasonal changes in the Western Namibian Savanna using spaceborne lidars}, series = {Remote sensing / Molecular Diversity Preservation International (MDPI)}, volume = {14}, journal = {Remote sensing / Molecular Diversity Preservation International (MDPI)}, number = {12}, edition = {12}, publisher = {MDPI}, address = {Basel, Schweiz}, issn = {2072-4292}, doi = {10.3390/rs14122928}, pages = {1 -- 20}, year = {2022}, abstract = {The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) with its land and vegetation height data product (ATL08), and Global Ecosystem Dynamics Investigation (GEDI) with its terrain elevation and height metrics data product (GEDI Level 2A) missions have great potential to globally map ground and canopy heights. Canopy height is a key factor in estimating above-ground biomass and its seasonal changes; these satellite missions can also improve estimated above-ground carbon stocks. This study presents a novel Sparse Vegetation Detection Algorithm (SVDA) which uses ICESat-2 (ATL03, geolocated photons) data to map tree and vegetation heights in a sparsely vegetated savanna ecosystem. The SVDA consists of three main steps: First, noise photons are filtered using the signal confidence flag from ATL03 data and local point statistics. Second, we classify ground photons based on photon height percentiles. Third, tree and grass photons are classified based on the number of neighbors. We validated tree heights with field measurements (n = 55), finding a root-mean-square error (RMSE) of 1.82 m using SVDA, GEDI Level 2A (Geolocated Elevation and Height Metrics product): 1.33 m, and ATL08: 5.59 m. Our results indicate that the SVDA is effective in identifying canopy photons in savanna ecosystems, where ATL08 performs poorly. We further identify seasonal vegetation height changes with an emphasis on vegetation below 3 m; widespread height changes in this class from two wet-dry cycles show maximum seasonal changes of 1 m, possibly related to seasonal grass-height differences. Our study shows the difficulties of vegetation measurements in savanna ecosystems but provides the first estimates of seasonal biomass changes.}, language = {en} } @incollection{Bookhagen2017, author = {Bookhagen, Bodo}, title = {The influence of hydrology and glaciology on wetlands in the Himalayas}, series = {Bird migration across the Himalayas : wetland functioning amidst mountains and glaciers}, booktitle = {Bird migration across the Himalayas : wetland functioning amidst mountains and glaciers}, editor = {Prins, Herbert H.T. and Namgail, Tsewang}, publisher = {Cambridge University Press}, address = {Cambridge}, isbn = {978-1-107-11471-5}, doi = {10.1017/9781316335420}, pages = {175 -- 188}, year = {2017}, abstract = {Birds migrating across the Himalayan region fly over the highest peaks in the world, facing immense physiological and climatic challenges. The authors show the different strategies used by birds to cope with these challenges. Many wetland avian species are seen in the high-altitude lakes of the Himalayas and the adjoining Tibetan Plateau, such as Bar-Headed Geese. Ringing programmes have generated information about origins and destinations, and this book is the first to present information on the bird's exact migratory paths. Capitalising on knowledge generated through satellite telemetry, the authors describe the migratory routes of a multitude of birds flying over or skirting the Himalayas. The myriad of threats to migratory birds and the wetland system in the Central Asian Flyway are discussed, with ways to mitigate them. This volume will inform and persuade policy-makers and conservation practitioners to take appropriate measures for the long-term survival of this unique migration}, language = {en} } @misc{SmithBookhagen2020, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Assessing Multi-Temporal Snow-Volume Trends in High Mountain Asia From 1987 to 2016 Using High-Resolution Passive Microwave Data}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1020}, issn = {1866-8372}, doi = {10.25932/publishup-48417}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-484176}, pages = {15}, year = {2020}, abstract = {High Mountain Asia (HMA) is dependent upon both the amount and timing of snow and glacier meltwater. Previous model studies and coarse resolution (0.25° × 0.25°, ∼25 km × 25 km) passive microwave assessments of trends in the volume and timing of snowfall, snowmelt, and glacier melt in HMA have identified key spatial and seasonal heterogeneities in the response of snow to changes in regional climate. Here we use recently developed, continuous, internally consistent, and high-resolution passive microwave data (3.125 km × 3.125 km, 1987-2016) from the special sensor microwave imager instrument family to refine and extend previous estimates of changes in the snow regime of HMA. We find an overall decline in snow volume across HMA; however, there exist spatially contiguous regions of increasing snow volume—particularly during the winter season in the Pamir, Karakoram, Hindu Kush, and Kunlun Shan. Detailed analysis of changes in snow-volume trends through time reveal a large step change from negative trends during the period 1987-1997, to much more positive trends across large regions of HMA during the periods 1997-2007 and 2007-2016. We also find that changes in high percentile monthly snow-water volume exhibit steeper trends than changes in low percentile snow-water volume, which suggests a reduction in the frequency of high snow-water volumes in much of HMA. Regions with positive snow-water storage trends generally correspond to regions of positive glacier mass balances.}, language = {en} } @article{LoiblBookhagenValadeetal.2019, author = {Loibl, David and Bookhagen, Bodo and Valade, Sebastien and Schneider, Christoph}, title = {OSARIS, the "Open Source SAR Investigation System" for Automatized Parallel InSAR Processing of Sentinel-1 Time Series Data With Special Emphasis on Cryosphere Applications}, series = {Frontiers in Earth Science}, volume = {7}, journal = {Frontiers in Earth Science}, publisher = {Frontiers Media}, address = {Lausanne}, issn = {2296-6463}, doi = {10.3389/feart.2019.00172}, pages = {20}, year = {2019}, abstract = {With the advent of the two Sentinel-1 (S1) satellites, Synthetic Aperture Radar (SAR) data with high temporal and spatial resolution are freely available. This provides a promising framework to facilitate detailed investigations of surface instabilities and movements on large scales with high temporal resolution, but also poses substantial processing challenges because of storage and computation requirements. Methods are needed to efficiently detect short term changes in dynamic environments. Approaches considering pair-wise processing of a series of consecutive scenes to retain maximum temporal resolution in conjunction with time series analyses are required. Here we present OSARIS, the "Open Source SAR Investigation System," as a framework to process large stacks of S1 data on high-performance computing clusters. Based on Generic Mapping Tools SAR, shell scripts, and the workload manager Slurm, OSARIS provides an open and modular framework combining parallelization of high-performance C programs, flexible processing schemes, convenient configuration, and generation of geocoded stacks of analysis-ready base data, including amplitude, phase, coherence, and unwrapped interferograms. Time series analyses can be conducted by applying automated modules to the data stacks. The capabilities of OSARIS are demonstrated in a case study from the northwestern Tien Shan, Central Asia. After merging of slices, a total of 80 scene pairs were processed from 174 total input scenes. The coherence time series exhibits pronounced seasonal variability, with relatively high coherence values prevailing during the summer months in the nival zone. As an example of a time series analysis module, we present OSARIS' "Unstable Coherence Metric" which identifies pixels affected by significant drops from high to low coherence values. Measurements of motion provided by LOSD measurements require careful evaluation because interferometric phase unwrapping is prone to errors. Here, OSARIS provides a series of modules to detect and mask unwrapping errors, correct for atmospheric disturbances, and remove large-scale trends. Wall clock processing time for the case study (area ~9,000 km2) was ~12 h 4 min on a machine with 400 cores and 2 TB RAM. In total, ~12 d 10 h 44 min (~96\%) were saved through parallelization. A comparison of selected OSARIS datasets to results from two state-of-the-art SAR processing suites, ISCE and SNAP, shows that OSARIS provides products of competitive quality despite its high level of automatization. OSARIS thus facilitates efficient S1-based region-wide investigations of surface movement events over multiple years.}, language = {en} } @article{WeldeabRuehlemannBookhagenetal.2019, author = {Weldeab, Syee and R{\"u}hlemann, Carsten and Bookhagen, Bodo and Pausata, Francesco S. R. and Perez-Lua, Fabiola M.}, title = {Enhanced Himalayan glacial melting during YD and H1 recorded in the Northern Bay of Bengal}, series = {Geochemistry, geophysics, geosystems}, volume = {20}, journal = {Geochemistry, geophysics, geosystems}, number = {5}, publisher = {American Geophysical Union}, address = {Washington}, issn = {1525-2027}, doi = {10.1029/2018GC008065}, pages = {2449 -- 2461}, year = {2019}, abstract = {Ocean-land thermal feedback mechanisms in the Indian Summer Monsoon (ISM) domain are an important but not well understood component of regional climate dynamics. Here we present a O-18 record analyzed in the mixed-layer dwelling planktonic foraminifer Globigerinoides ruber (sensu stricto) from the northernmost Bay of Bengal (BoB). The O-18 time series provides a spatially integrated measure of monsoonal precipitation and Himalayan meltwater runoff into the northern BoB and reveals two brief episodes of anomalously low O-18 values between 16.30.4 and 160.5 and 12.60.4 and 12.30.4 thousand years before present. The timing of these events is centered at Heinrich event 1 and the Younger Dryas, well-known phases of weak northern hemisphere monsoon systems. Numerical climate model experiments, simulating Heinrich event-like conditions, suggest a surface warming over the monsoon-dominated Himalaya and foreland in response to ISM weakening. Corroborating the simulation results, our analysis of published moraine exposure ages in the monsoon-dominated Himalaya indicates enhanced glacier retreats that, considering age model uncertainties, coincide and overlap with the episodes of anomalously low O-18 values in the northernmost BoB. Our climate proxy and simulation results provide insights into past regional climate dynamics, suggesting reduced cloud cover, increased solar radiation, and air warming of the Himalaya and foreland areas and, as a result, glacier mass losses in response to weakened ISM. Plain Language Summary Indian Summer Monsoon rainfall and Himalayan glacier/snow melts constitute the main water source for the densely populated Indian subcontinent. Better understanding of how future climate changes will affect the monsoon rainfall and Himalayan glaciers requires a long climate record. In this study, we create a 13,000-year-long climate record that allows us to better understand the response of Indian Summer Monsoon rainfall and Himalayan glaciers to past climate changes. The focus of our study is the time window between 9,000 and 22,000 years ago, an episode where the global climate experienced large and rapid changes. Our sediment record from the northern Bay of Bengal and climate change simulation indicate that during episodes of weak monsoon, the melting of the Himalayan glaciers increases substantially significantly. This is because the weakening of the monsoon results in less cloud cover and, as a result, the surface receives more sunlight and causes glacier melting.}, language = {en} } @article{MerchelGaertnerBeutneretal.2019, author = {Merchel, Silke and G{\"a}rtner, Andreas and Beutner, Sabrina and Bookhagen, Bodo and Chabilan, Amelie}, title = {Attempts to understand potential deficiencies in chemical procedures for AMS: Cleaning and dissolving quartz for Be-10 and Al-26 analysis}, series = {Nuclear instruments \& methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics, Section B, Beam interactions with materials and atoms}, volume = {455}, journal = {Nuclear instruments \& methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics, Section B, Beam interactions with materials and atoms}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0168-583X}, doi = {10.1016/j.nimb.2019.02.007}, pages = {293 -- 299}, year = {2019}, abstract = {The purity of the analysed samples (e.g. quartz) with respect to chemical composition and radionuclide contamination is essential for geomorphologic applications using so-called terrestrial cosmogenic nuclides (TCNs). To guarantee this, numerous cleaning and dissolution procedures have been developed. At the DREsden Accelerator Mass Spectrometry (DREAMS) facility, we also work on enhancing the chemical quartz-enrichment methodology from bulk rock and dissolution of quartz. Repeated exposure of the bulk material to acid mixtures (HCl/H2SiF6) at room temperature for cleaning and its monitoring by optical microscopy works for most quartz-rich samples. The quartz dissolution in HF under rather mild conditions (at room temperature on a shaker-table) has the advantage to leave difficult-to-dissolve minerals (e.g., tourmaline, zircon, rutile, sillimanite, kyanite, chromite, corundum), not separated by other physical methods before, as residue. Our comparison with a high-temperature dissolution method (in a microwave) indicates an additional amount of interfering elements, such as in average about 3 mg of Ti, more than 7 mg of Al, and about 22 mu g of Be (for 50 g SiO2), is added to the sample, hence showing the superiority of our mild method. This way, we reduce problems for chemistry and AMS, but also ensure better comparability to production rates of cleaner stoichiometric quartz from calibration sites.}, language = {en} } @article{ClubbBookhagenRheinwalt2019, author = {Clubb, Fiona J. and Bookhagen, Bodo and Rheinwalt, Aljoscha}, title = {Clustering river profiles to classify geomorphic domains}, series = {Journal of geophysical research : Earth surface}, volume = {124}, journal = {Journal of geophysical research : Earth surface}, number = {6}, publisher = {American Geophysical Union}, address = {Hoboken}, issn = {2169-9003}, doi = {10.1029/2019JF005025}, pages = {1417 -- 1439}, year = {2019}, abstract = {The structure and organization of river networks has been used for decades to investigate the influence of climate and tectonics on landscapes. The majority of these studies either analyze rivers in profile view by extracting channel steepness or calculate planform metrics such as drainage density. However, these techniques rely on the assumption of homogeneity: that intrinsic and external factors are spatially or temporally invariant over the measured profile. This assumption is violated for the majority of Earth's landscapes, where variations in uplift rate, rock strength, climate, and geomorphic process are almost ubiquitous. We propose a method for classifying river profiles to identify landscape regions with similar characteristics by adapting hierarchical clustering algorithms developed for time series data. We first test our clustering on two landscape evolution scenarios and find that we can successfully cluster regions with different erodibility and detect the transient response to sudden base level fall. We then test our method in two real landscapes: first in Bitterroot National Forest, Idaho, where we demonstrate that our method can detect transient incision waves and the topographic signature of fluvial and debris flow process regimes; and second, on Santa Cruz Island, California, where our technique identifies spatial patterns in lithology not detectable through normalized channel steepness analysis. By calculating channel steepness separately for each cluster, our method allows the extraction of more reliable steepness metrics than if calculated for the landscape as a whole. These examples demonstrate the method's ability to disentangle fluvial morphology in complex lithological and tectonic settings.}, language = {en} } @article{RheinwahltGoswamiBookhagen2019, author = {Rheinwahlt, Aljoscha and Goswami, Bedartha and Bookhagen, Bodo}, title = {A network-based flow accumulation algorithm for point clouds}, series = {Journal of geophysical research : Earth surface}, volume = {124}, journal = {Journal of geophysical research : Earth surface}, number = {7}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1029/2018JF004827}, pages = {2013 -- 2033}, year = {2019}, abstract = {Flow accumulation algorithms estimate the steady state of flow on real or modeled topographic surfaces and are crucial for hydrological and geomorphological assessments, including delineation of river networks, drainage basins, and sediment transport processes. Existing flow accumulation algorithms are typically designed to compute flows on regular grids and are not directly applicable to arbitrarily sampled topographic data such as lidar point clouds. In this study we present a random sampling scheme that generates homogeneous point densities, in combination with a novel flow path tracing approach-the Facet-Flow Network (FFN)-that estimates flow accumulation in terms of specific catchment area (SCA) on triangulated surfaces. The random sampling minimizes biases due to spatial sampling and the FFN allows for direct flow estimation from point clouds. We validate our approach on a Gaussian hill surface and study the convergence of its SCA compared to the analytical solution. Here, our algorithm outperforms the multiple flow direction algorithm, which is optimized for divergent surfaces. We also compute the SCA of a 6-km(2)-steep, vegetated catchment on Santa Cruz Island, California, based on airborne lidar point-cloud data. Point-cloud-based SCA values estimated by our method compare well with those estimated by the D-infinity or multiple flow direction algorithm on gridded data. The advantage of computing SCA from point clouds becomes relevant especially for divergent topography and for small drainage areas: These are depicted with much more detail due to the higher sampling density of point clouds.}, language = {en} } @article{SmithBookhagen2020, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Assessing Multi-Temporal Snow-Volume Trends in High Mountain Asia From 1987 to 2016 Using High-Resolution Passive Microwave Data}, series = {Frontiers in Earth Science}, volume = {8}, journal = {Frontiers in Earth Science}, publisher = {Frontiers Media}, address = {Lausanne}, issn = {2296-6463}, doi = {10.3389/feart.2020.559175}, pages = {13}, year = {2020}, abstract = {High Mountain Asia (HMA) is dependent upon both the amount and timing of snow and glacier meltwater. Previous model studies and coarse resolution (0.25° × 0.25°, ∼25 km × 25 km) passive microwave assessments of trends in the volume and timing of snowfall, snowmelt, and glacier melt in HMA have identified key spatial and seasonal heterogeneities in the response of snow to changes in regional climate. Here we use recently developed, continuous, internally consistent, and high-resolution passive microwave data (3.125 km × 3.125 km, 1987-2016) from the special sensor microwave imager instrument family to refine and extend previous estimates of changes in the snow regime of HMA. We find an overall decline in snow volume across HMA; however, there exist spatially contiguous regions of increasing snow volume—particularly during the winter season in the Pamir, Karakoram, Hindu Kush, and Kunlun Shan. Detailed analysis of changes in snow-volume trends through time reveal a large step change from negative trends during the period 1987-1997, to much more positive trends across large regions of HMA during the periods 1997-2007 and 2007-2016. We also find that changes in high percentile monthly snow-water volume exhibit steeper trends than changes in low percentile snow-water volume, which suggests a reduction in the frequency of high snow-water volumes in much of HMA. Regions with positive snow-water storage trends generally correspond to regions of positive glacier mass balances.}, language = {en} } @misc{OlenBookhagen2018, author = {Olen, Stephanie M. and Bookhagen, Bodo}, title = {Mapping Damage-Affected Areas after Natural Hazard Events Using Sentinel-1 Coherence Time Series}, series = {remote sensing}, journal = {remote sensing}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-417766}, pages = {19}, year = {2018}, abstract = {The emergence of the Sentinel-1A and 1B satellites now offers freely available and widely accessible Synthetic Aperture Radar (SAR) data. Near-global coverage and rapid repeat time (6-12 days) gives Sentinel-1 data the potential to be widely used for monitoring the Earth's surface. Subtle land-cover and land surface changes can affect the phase and amplitude of the C-band SAR signal, and thus the coherence between two images collected before and after such changes. Analysis of SAR coherence therefore serves as a rapidly deployable and powerful tool to track both seasonal changes and rapid surface disturbances following natural disasters. An advantage of using Sentinel-1 C-band radar data is the ability to easily construct time series of coherence for a region of interest at low cost. In this paper, we propose a new method for Potentially Affected Area (PAA) detection following a natural hazard event. Based on the coherence time series, the proposed method (1) determines the natural variability of coherence within each pixel in the region of interest, accounting for factors such as seasonality and the inherent noise of variable surfaces; and (2) compares pixel-by-pixel syn-event coherence to temporal coherence distributions to determine where statistically significant coherence loss has occurred. The user can determine to what degree the syn-event coherence value (e.g., 1st, 5th percentile of pre-event distribution) constitutes a PAA, and integrate pertinent regional data, such as population density, to rank and prioritise PAAs. We apply the method to two case studies, Sarpol-e, Iran following the 2017 Iran-Iraq earthquake, and a landslide-prone region of NW Argentina, to demonstrate how rapid identification and interpretation of potentially affected areas can be performed shortly following a natural hazard event.}, language = {en} } @article{NeelyBookhagenBurbank2017, author = {Neely, Alexander B. and Bookhagen, Bodo and Burbank, Douglas W.}, title = {An automated knickzone selection algorithm (KZ-Picker) to analyze transient landscapes: Calibration and validation}, series = {Journal of geophysical research : Earth surface}, volume = {122}, journal = {Journal of geophysical research : Earth surface}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1002/2017JF004250}, pages = {1236 -- 1261}, year = {2017}, abstract = {Streams commonly respond to base-level fall by localizing erosion within steepened, convex knickzone reaches. Localized incision causes knickzone reaches to migrate upstream. Such migrating knickzones dictate the pace of landscape response to changes in tectonics or erosional efficiency and can help quantify the timing and source of base-level fall. Identification of knickzones typically requires individual selection of steepened reaches: a process that is tedious and subjective and has no efficient means to measure knickzone size. We construct an algorithm to automate this procedure by selecting the bounds of knickzone reaches in a -space (drainage-area normalized) framework. An automated feature calibrates algorithm parameters to a subset of knickzones handpicked by the user. The algorithm uses these parameters as consistent criteria to identify knickzones objectively, and then the algorithm measures the height, length, and slope of each knickzone reach. We test the algorithm on 1, 10, and 30m resolution digital elevation models (DEMs) of six catchments (trunk-stream lengths: 2.1-5.4km) on Santa Cruz Island, southern California. On the 1m DEM, algorithm-selected knickzones confirm 93\% of handpicked knickzone positions (n=178) to a spatial accuracy of 100m, 88\% to an accuracy within 50m, and 46\% to an accuracy within 10m. Using 10 and 30m DEMs, accuracy is similar: 88-86\% to 100m and 82\% to 50m (n=38 and 36, respectively). The algorithm enables efficient regional comparison of the size and location of knickzones with geologic structures, mapped landforms, and hillslope morphology, thereby facilitating approaches to characterize the dynamics of transient landscapes. Plain Language Summary The shape of rivers reflects the environments that they flow through and the environments that they link together: mountains and oceans. Anywhere along the length of a river, changes in environmental conditions are propagated upstream and downstream as the river changes its morphology to match the new environmental conditions. Commonly, rivers steepen as land uplifts faster in regions of high tectonic convergence. The steepening of river gradients is propagated upstream and can be mapped to trace zones of high tectonic activity across landscapes and estimate the source and timing of environmental change. Such insights may indicate regions where earthquakes have become more frequent in the recent past and how rivers respond to these changes. In this submission, we detail an algorithm that can use digital topographic data (similar to google earth), to automatically map and measure anomalously steep river reaches across continental scales. This technology can highlight areas that have experienced recent sustained changes in environmental conditions, evident by changes in the morphology of rivers. Such environmental conditions could be changes in tectonic uplift and earthquake activity, changes in sea level, changes in land-use, or changes in climate, all factors that can produce measurable differences in river morphology over time.}, language = {en} } @article{SmithBookhagenRheinwalt2017, author = {Smith, Taylor and Bookhagen, Bodo and Rheinwalt, Aljoscha}, title = {identified with an automated snowmelt detection algorithm, 1987-2016}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {11}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1994-0416}, doi = {10.5194/tc-11-2329-2017}, pages = {2329 -- 2343}, year = {2017}, abstract = {High Mountain Asia (HMA) - encompassing the Tibetan Plateau and surrounding mountain ranges - is the primary water source for much of Asia, serving more than a billion downstream users. Many catchments receive the majority of their yearly water budget in the form of snow, which is poorly monitored by sparse in situ weather networks. Both the timing and volume of snowmelt play critical roles in downstream water provision, as many applications - such as agriculture, drinking-water generation, and hydropower - rely on consistent and predictable snowmelt runoff. Here, we examine passive microwave data across HMA with five sensors (SSMI, SSMIS, AMSR-E, AMSR2, and GPM) from 1987 to 2016 to track the timing of the snowmelt season - defined here as the time between maximum passive microwave signal separation and snow clearance. We validated our method against climate model surface temperatures, optical remote-sensing snow-cover data, and a manual control dataset (n\&\#8201;=\&\#8201;2100, 3 variables at 25 locations over 28 years); our algorithm is generally accurate within 3-5 days. Using the algorithm-generated snowmelt dates, we examine the spatiotemporal patterns of the snowmelt season across HMA. The climatically short (29-year) time series, along with complex interannual snowfall variations, makes determining trends in snowmelt dates at a single point difficult. We instead identify trends in snowmelt timing by using hierarchical clustering of the passive microwave data to determine trends in self-similar regions. We make the following four key observations. (1) The end of the snowmelt season is trending almost universally earlier in HMA (negative trends). Changes in the end of the snowmelt season are generally between 2 and 8 days decade\&\#8722;1 over the 29-year study period (5-25 days total). The length of the snowmelt season is thus shrinking in many, though not all, regions of HMA. Some areas exhibit later peak signal separation (positive trends), but with generally smaller magnitudes than trends in snowmelt end. (2) Areas with long snowmelt periods, such as the Tibetan Plateau, show the strongest compression of the snowmelt season (negative trends). These trends are apparent regardless of the time period over which the regression is performed. (3) While trends averaged over 3 decades indicate generally earlier snowmelt seasons, data from the last 14 years (2002-2016) exhibit positive trends in many regions, such as parts of the Pamir and Kunlun Shan. Due to the short nature of the time series, it is not clear whether this change is a reversal of a long-term trend or simply interannual variability. (4) Some regions with stable or growing glaciers - such as the Karakoram and Kunlun Shan - see slightly later snowmelt seasons and longer snowmelt periods. It is likely that changes in the snowmelt regime of HMA account for some of the observed heterogeneity in glacier response to climate change. While the decadal increases in regional temperature have in general led to earlier and shortened melt seasons, changes in HMA's cryosphere have been spatially and temporally heterogeneous.}, language = {en} } @article{BufeBurbankLiuetal.2017, author = {Bufe, Aaron and Burbank, Douglas W. and Liu, Langtao and Bookhagen, Bodo and Qin, Jintang and Chen, Jie and Li, Tao and Jobe, Jessica Ann Thompson and Yang, Huili}, title = {Variations of Lateral Bedrock Erosion Rates Control Planation of Uplifting Folds in the Foreland of the Tian Shan, NW China}, series = {Journal of geophysical research : Earth surface}, volume = {122}, journal = {Journal of geophysical research : Earth surface}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1002/2016JF004099}, pages = {2431 -- 2467}, year = {2017}, abstract = {Fluvial planation surfaces, such as straths, commonly serve as recorders of climatic and tectonic changes and are formed by the lateral erosion of rivers, a process that remains poorly understood. Here we present a study of kilometer-wide, fluvially eroded, low-relief surfaces on rapidly uplifting folds in the foreland of the southwestern Tian Shan. A combination of field work, digital elevation model analysis, and dating of fluvial deposits reveals that despite an arid climate and rapid average rock-uplift rates of 1-3mm/yr, rivers cut extensive (>1-2km wide) surfaces with typical height variations of <6m over periods of >2-6kyr. The extent of this beveling varies in space and time, such that different beveling episodes affect individual structures. Between times of planation, beveled surfaces are abandoned, incised, and deformed across the folds. In a challenge to models that link strath cutting and abandonment primarily to changes in river incision rates, we demonstrate that lateral erosion rates of antecedent streams crossing the folds have to vary by more than 1 order of magnitude to explain the creation of beveled platforms in the past and their incision at the present day. These variations do not appear to covary with climate variability and might be caused by relatively small (much less than an order of magnitude) changes in sediment or water fluxes. It remains uncertain in which settings variations in lateral bedrock erosion rates predominate over changes in vertical erosion rates. Therefore, when studying fluvial planation and strath terraces, variability of both lateral and vertical erosion rates should be considered.}, language = {en} } @article{NorrisCarvalhoJonesetal.2017, author = {Norris, Jesse and Carvalho, Leila M. V. and Jones, Charles and Cannon, Forest and Bookhagen, Bodo and Palazzi, Elisa and Tahir, Adnan Ahmad}, title = {The spatiotemporal variability of precipitation over the Himalaya: evaluation of one-year WRF model simulation}, series = {Climate dynamics : observational, theoretical and computational research on the climate system}, volume = {49}, journal = {Climate dynamics : observational, theoretical and computational research on the climate system}, publisher = {Springer}, address = {New York}, issn = {0930-7575}, doi = {10.1007/s00382-016-3414-y}, pages = {2179 -- 2204}, year = {2017}, abstract = {The Weather Research and Forecasting (WRF) model is used to simulate the spatiotemporal distribution of precipitation over central Asia over the year April 2005 through March 2006. Experiments are performed at 6.7 km horizontal grid spacing, with an emphasis on winter and summer precipitation over the Himalaya. The model and the Tropical Rainfall Measuring Mission show a similar inter-seasonal cycle of precipitation, from extratropical cyclones to monsoon precipitation, with agreement also in the diurnal cycle of monsoon precipitation. In winter months, WRF compares better in timeseries of daily precipitation to stations below than above 3-km elevation, likely due to inferior measurement of snow than rain by the stations, highlighting the need for reliable snowfall measurements at high elevations in winter. In summer months, the nocturnal precipitation cycle in the foothills and valleys of the Himalaya is captured by this 6.7-km WRF simulation, while coarser simulations with convective parameterization show near zero nocturnal precipitation. In winter months, higher resolution is less important, serving only to slightly increase precipitation magnitudes due to steeper slopes. However, even in the 6.7-km simulation, afternoon precipitation is overestimated at high elevations, which can be reduced by even higher-resolution (2.2-km) simulations. These results indicate that WRF provides skillful simulations of precipitation relevant for studies of water resources over the complex terrain in the Himalaya.}, language = {en} } @article{BufeBekaertHussainetal.2017, author = {Bufe, Aaron and Bekaert, David P. S. and Hussain, Ekbal and Bookhagen, Bodo and Burbank, Douglas W. and Jobe, Jessica Ann Thompson and Chen, Jie and Li, Tao and Liu, Langtao and Gan, Weijun}, title = {Temporal changes in rock uplift rates of folds in the foreland of the Tian Shan and the Pamir from geodetic and geologic data}, series = {Geophysical research letters}, volume = {44}, journal = {Geophysical research letters}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1002/2017GL073627}, pages = {10977 -- 10987}, year = {2017}, abstract = {Understanding the evolution of continental deformation zones relies on quantifying spatial and temporal changes in deformation rates of tectonic structures. Along the eastern boundary of the Pamir-Tian Shan collision zone, we constrain secular variations of rock uplift rates for a series of five Quaternary detachment- and fault-related folds from their initiation to the modern day. When combined with GPS data, decomposition of interferometric synthetic aperture radar time series constrains the spatial pattern of surface and rock uplift on the folds deforming at decadal rates of 1-5mm/yr. These data confirm the previously proposed basinward propagation of structures during the Quaternary. By fitting our geodetic rates and previously published geologic uplift rates with piecewise linear functions, we find that gradual rate changes over >100kyr can explain the interferometric synthetic aperture radar observations where changes in average uplift rates are greater than similar to 1 mm/yr among different time intervals (similar to 10(1), 10(4-5), and 10(5-6) years).}, language = {en} } @article{NeelmeijerMotaghBookhagen2017, author = {Neelmeijer, Julia and Motagh, Mandi and Bookhagen, Bodo}, title = {High-resolution digital elevation models from single-pass TanDEM-X interferometry over mountainous regions: A case study of Inylchek Glacier, Central Asia}, series = {ISPRS journal of photogrammetry and remote sensing : official publication of the International Society for Photogrammetry and Remote Sensing}, volume = {130}, journal = {ISPRS journal of photogrammetry and remote sensing : official publication of the International Society for Photogrammetry and Remote Sensing}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0924-2716}, doi = {10.1016/j.isprsjprs.2017.05.011}, pages = {108 -- 121}, year = {2017}, abstract = {This study demonstrates the potential of using single-pass TanDEM-X (TDX) radar imagery to analyse inter- and intra-annual glacier changes in mountainous terrain. Based on SAR images acquired in February 2012, March 2013 and November 2013 over the Inylchek Glacier, Kyrgyzstan, we discuss in detail the processing steps required to generate three reliable digital elevation models (DEMs) with a spatial resolution of 10 m that can be used for glacial mass balance studies. We describe the interferometric processing steps and the influence of a priori elevation information that is required to model long wavelength topographic effects. We also focus on DEM alignment to allow optimal DEM comparisons and on the effects of radar signal penetration on ice and snow surface elevations. We finally compare glacier elevation changes between the three TDX DEMs and the C-band shuttle radar topography mission (SRTM) DEM from February 2000. We introduce a new approach for glacier elevation change calculations that depends on the elevation and slope of the terrain. We highlight the superior quality of the TDX DEMs compared to the SRTM DEM, describe remaining DEM uncertainties and discuss the limitations that arise due to the side-looking nature of the radar sensor. (C) 2017 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS). Published by Elsevier B.V. All rights reserved.}, language = {en} } @misc{SmithBookhagenCannon2015, author = {Smith, Taylor and Bookhagen, Bodo and Cannon, Forest}, title = {Improving semi-automated glacier mapping with a multi-method approach}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {510}, issn = {1866-8372}, doi = {10.25932/publishup-40847}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-408471}, pages = {13}, year = {2015}, abstract = {Studies of glaciers generally require precise glacier outlines. Where these are not available, extensive manual digitization in a geographic information system (GIS) must be performed, as current algorithms struggle to delineate glacier areas with debris cover or other irregular spectral profiles. Although several approaches have improved upon spectral band ratio delineation of glacier areas, none have entered wide use due to complexity or computational intensity. In this study, we present and apply a glacier mapping algorithm in Central Asia which delineates both clean glacier ice and debris-covered glacier tongues. The algorithm is built around the unique velocity and topographic characteristics of glaciers and further leverages spectral and spatial relationship data. We found that the algorithm misclassifies between 2 and 10\% of glacier areas, as compared to a similar to 750 glacier control data set, and can reliably classify a given Landsat scene in 3-5 min. The algorithm does not completely solve the difficulties inherent in classifying glacier areas from remotely sensed imagery but does represent a significant improvement over purely spectral-based classification schemes, such as the band ratio of Landsat 7 bands three and five or the normalized difference snow index. The main caveats of the algorithm are (1) classification errors at an individual glacier level, (2) reliance on manual intervention to separate connected glacier areas, and (3) dependence on fidelity of the input Landsat data.}, language = {en} } @article{SmithBookhagen2016, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Assessing uncertainty and sensor biases in passive microwave data across High Mountain Asia}, series = {Remote sensing of environment : an interdisciplinary journal}, volume = {181}, journal = {Remote sensing of environment : an interdisciplinary journal}, publisher = {Elsevier}, address = {New York}, issn = {0034-4257}, doi = {10.1016/j.rse.2016.03.037}, pages = {174 -- 185}, year = {2016}, abstract = {Snowfall comprises a significant percentage of the annual water budget in High Mountain Asia (HMA), but snow water equivalent (SWE) is poorly constrained due to lack of in-situ measurements and complex terrain that limits the efficacy of modeling and observations. Over the past few decades, SWE has been estimated with passive microwave (PM) sensors with generally good results in wide, flat, terrain, and lower reliability in densely forested, complex, or high-elevation areas. In this study, we use raw swath data from five satellite - sensors the Special Sensor Microwave/Imager (SSMI) and Special Sensor Microwave Imager/Sounder (SSMIS) (1987-2015, F08, F11, F13, F17), Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E, 2002-2011), AMSR2 (2012-2015), and the Global Precipitation Measurement (GPM, 2014-2015) - in order to understand the spatial and temporal structure of native sensor, topographic, and land cover biases in SWE estimates in HMA. We develop a thorough understanding of the uncertainties in our SWE estimates by examining the impacts of topographic parameters (aspect, relief, hillslope angle, and elevation), land cover, native sensor biases, and climate parameters (precipitation, temperature, and wind speed). HMA, with its high seasonality, large topographic gradients and low relief at high elevations provides an excellent context to examine a wide range of climatic, land-cover, and topographic settings to better constrain SWE uncertainties and potential sensor bias. Using a multi-parameter regression, we compare long-term SWE variability to forest fraction, maximal multiyear snow depth, topographic parameters, and long-term average wind speed across both individual sensor time series and a merged multi-sensor dataset. In regions where forest cover is extensive, it is the strongest control on SWE variability. In those regions where forest density is low (<5\%), maximal snow depth dominates the uncertainty signal. In our regression across HMA, we find that forest fraction is the strongest control on SWE variability (75.8\%), followed by maximal multi-year snow depth (7.82\%), 90th percentile 10-m wind speed of a 10-year December-January-February (DJF) time series (5.64\%), 25th percentile DJF 10-m wind speed (5.44\%), and hillslope angle (5.24\%). Elevation, relief, and terrain aspect show very low influence on SWE variability (<1\%). We find that the GPM sensor provides the most robust regression results, and can be reliably used to estimate SWE in our study region. While forest cover and elevation have been integrated into many SWE algorithms, wind speed and long-term maximal snow depth have not. Our results show that wind redistribution of snow can have impacts on SWE, especially over large, flat, areas. Using our regression results, we have developed an understanding of sensor specific SWE uncertainties and their spatial patterns. The uncertainty maps developed in this study provide a first-order approximation of SWE-estimate reliability for much of HMA, and imply that high-fidelity SWE estimates can be produced for many high-elevation areas. (C) 2016 Elsevier Inc. All rights reserved.}, language = {en} } @article{WulfBookhagenScherler2016, author = {Wulf, Hendrik and Bookhagen, Bodo and Scherler, Dirk}, title = {Differentiating between rain, snow, and glacier contributions to river discharge in the western Himalaya using remote-sensing data and distributed hydrological modeling}, series = {Advances in water resources}, volume = {88}, journal = {Advances in water resources}, publisher = {Elsevier}, address = {Oxford}, issn = {0309-1708}, doi = {10.1016/j.advwatres.2015.12.004}, pages = {152 -- 169}, year = {2016}, abstract = {Rivers draining the southern Himalaya provide most of the water supply for the densely populated Indo-Gangetic plains. Despite the importance of water resources in light of climate change, the relative contributions of rainfall, snow and glacier melt to discharge are not well understood, due to the scarcity of ground-based data in this complex terrain. Here, we quantify discharge sources in the Sutlej Valley, western Himalaya, from 2000 to 2012 with a distributed hydrological model that is based on daily, ground-calibrated remote-sensing observation. Based on the consistently good model performance, we analyzed the spatiotemporal distribution of hydrologic components and quantified their contribution to river discharge. Our results indicate that the Sutlej River's annual discharge at the mountain front is sourced to 55\% by effective rainfall (rainfall reduced by evapotranspiration), 35\% by snow melt and 10\% by glacier melt. In the high-elevation orogenic interior glacial runoff contributes \&\#8764;30\% to annual river discharge. These glacier melt contributions are especially important during years with substantially reduced rainfall and snowmelt runoff, as during 2004, to compensate for low river discharge and ensure sustained water supply and hydropower generation. In 2004, discharge of the Sutlej River totaled only half the maximum annual discharge; with 17.3\% being sourced by glacier melt. Our findings underscore the importance of calibrating remote-sensing data with ground-based data to constrain hydrological models with reasonable accuracy. For instance, we found that TRMM (Tropical Rainfall Measuring Mission) product 3B42 V7 systematically overestimates rainfall in arid regions of our study area by a factor of up to 5. By quantifying the spatiotemporal distribution of water resources we provide an important assessment of the potential impact of global warming on river discharge in the western Himalaya. Given the near-global coverage of the utilized remote-sensing datasets this hydrological modeling approach can be readily transferred to other data-sparse regions.}, language = {en} } @article{ForteWhippleBookhagenetal.2016, author = {Forte, Adam M. and Whipple, Kelin X. and Bookhagen, Bodo and Rossi, Matthew W.}, title = {Decoupling of modern shortening rates, climate, and topography in the Caucasus}, series = {Earth \& planetary science letters}, volume = {449}, journal = {Earth \& planetary science letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-821X}, doi = {10.1016/j.epsl.2016.06.013}, pages = {282 -- 294}, year = {2016}, abstract = {The Greater and Lesser Caucasus mountains and their associated foreland basins contain similar rock types, experience a similar two-fold, along-strike variation in mean annual precipitation, and were affected by extreme base-level drops of the neighboring Caspian Sea. However, the two Caucasus ranges are characterized by decidedly different tectonic regimes and rates of deformation that are subject to moderate (less than an order of magnitude) gradients in climate, and thus allow for a unique opportunity to isolate the effects of climate and tectonics in the evolution of topography within active orogens. There is an apparent disconnect between modern climate, shortening rates, and topography of both the Greater Caucasus and Lesser Caucasus which exhibit remarkably similar topography along-strike despite the gradients in forcing. By combining multiple datasets, we examine plausible causes for this disconnect by presenting a detailed analysis of the topography of both ranges utilizing established relationships between catchment-mean erosion rates and topography (local relief, hillslope gradients, and channel steepness) and combining it with a synthesis of previously published low-temperature thermochronologic data. Modern climate of the Caucasus region is assessed through an analysis of remotely-sensed data (TRMM and MODIS) and historical streamflow data. Because along-strike variation in either erosional efficiency or thickness of accreted material fail to explain our observations, we suggest that the topography of both the western Lesser and Greater Caucasus are partially supported by different geodynamic forces. In the western Lesser Caucasus, high relief portions of the landscape likely reflect uplift related to ongoing mantle lithosphere delamination beneath the neighboring East Anatolian Plateau. In the Greater Caucasus, maintenance of high topography in the western portion of the range despite extremely low (<2-4 mm/y) modern convergence rates may be related to dynamic topography from detachment of the north-directed Greater Caucasus slab or to a recent slowing of convergence rates. Large-scale spatial gradients in climate are not reflected in the topography of the Caucasus and do not seem to exert any significant control on the tectonics or structure of either range. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} } @article{MalikBookhagenMucha2016, author = {Malik, Nishant and Bookhagen, Bodo and Mucha, Peter J.}, title = {Spatiotemporal patterns and trends of Indian monsoonal rainfall extremes}, series = {Geophysical research letters}, volume = {43}, journal = {Geophysical research letters}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1002/2016GL067841}, pages = {1710 -- 1717}, year = {2016}, abstract = {In this study, we provide a comprehensive analysis of trends in the extremes during the Indian summer monsoon (ISM) months (June to September) at different temporal and spatial scales. Our goal is to identify and quantify spatiotemporal patterns and trends that have emerged during the recent decades and may be associated with changing climatic conditions. Our analysis primarily relies on quantile regression that avoids making any subjective choices on spatial, temporal, or intensity pattern of extreme rainfall events. Our analysis divides the Indian monsoon region into climatic compartments that show different and partly opposing trends. These include strong trends toward intensified droughts in Northwest India, parts of Peninsular India, and Myanmar; in contrast, parts of Pakistan, Northwest Himalaya, and Central India show increased extreme daily rain intensity leading to higher flood vulnerability. Our analysis helps explain previously contradicting results of trends in average ISM rainfall.}, language = {en} } @article{PurintonBookhagen2021, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Beyond Vertical Point Accuracy}, series = {Frontiers in Earth Science}, journal = {Frontiers in Earth Science}, publisher = {Frontiers Media}, address = {Lausanne, Schweiz}, issn = {2296-6463}, doi = {10.3389/feart.2021.758606}, pages = {1 -- 24}, year = {2021}, abstract = {Quantitative geomorphic research depends on accurate topographic data often collected via remote sensing. Lidar, and photogrammetric methods like structure-from-motion, provide the highest quality data for generating digital elevation models (DEMs). Unfortunately, these data are restricted to relatively small areas, and may be expensive or time-consuming to collect. Global and near-global DEMs with 1 arcsec (∼30 m) ground sampling from spaceborne radar and optical sensors offer an alternative gridded, continuous surface at the cost of resolution and accuracy. Accuracy is typically defined with respect to external datasets, often, but not always, in the form of point or profile measurements from sources like differential Global Navigation Satellite System (GNSS), spaceborne lidar (e.g., ICESat), and other geodetic measurements. Vertical point or profile accuracy metrics can miss the pixel-to-pixel variability (sometimes called DEM noise) that is unrelated to true topographic signal, but rather sensor-, orbital-, and/or processing-related artifacts. This is most concerning in selecting a DEM for geomorphic analysis, as this variability can affect derivatives of elevation (e.g., slope and curvature) and impact flow routing. We use (near) global DEMs at 1 arcsec resolution (SRTM, ASTER, ALOS, TanDEM-X, and the recently released Copernicus) and develop new internal accuracy metrics to assess inter-pixel variability without reference data. Our study area is in the arid, steep Central Andes, and is nearly vegetation-free, creating ideal conditions for remote sensing of the bare-earth surface. We use a novel hillshade-filtering approach to detrend long-wavelength topographic signals and accentuate short-wavelength variability. Fourier transformations of the spatial signal to the frequency domain allows us to quantify: 1) artifacts in the un-projected 1 arcsec DEMs at wavelengths greater than the Nyquist (twice the nominal resolution, so > 2 arcsec); and 2) the relative variance of adjacent pixels in DEMs resampled to 30-m resolution (UTM projected). We translate results into their impact on hillslope and channel slope calculations, and we highlight the quality of the five DEMs. We find that the Copernicus DEM, which is based on a carefully edited commercial version of the TanDEM-X, provides the highest quality landscape representation, and should become the preferred DEM for topographic analysis in areas without sufficient coverage of higher-quality local DEMs.}, language = {en} } @misc{HussBookhagenHuggeletal.2017, author = {Huss, Matthias and Bookhagen, Bodo and Huggel, C. and Jacobsen, Dean and Bradley, Raymond S. and Clague, J. J. and Vuille, Mathias and Buytaert, Wouter and Cayan, D. R. and Greenwood, G. and Mark, B. G. and Milner, A. M. and Weingartner, Rolf and Winder, M.}, title = {Toward mountains without permanent snow and ice}, series = {Earths future}, volume = {5}, journal = {Earths future}, publisher = {Wiley}, address = {Hoboken}, issn = {2328-4277}, doi = {10.1002/2016EF000514}, pages = {418 -- 435}, year = {2017}, abstract = {The cryosphere in mountain regions is rapidly declining, a trend that is expected to accelerate over the next several decades due to anthropogenic climate change. A cascade of effects will result, extending from mountains to lowlands with associated impacts on human livelihood, economy, and ecosystems. With rising air temperatures and increased radiative forcing, glaciers will become smaller and, in some cases, disappear, the area of frozen ground will diminish, the ratio of snow to rainfall will decrease, and the timing and magnitude of both maximum and minimum streamflow will change. These changes will affect erosion rates, sediment, and nutrient flux, and the biogeochemistry of rivers and proglacial lakes, all of which influence water quality, aquatic habitat, and biotic communities. Changes in the length of the growing season will allow low-elevation plants and animals to expand their ranges upward. Slope failures due to thawing alpine permafrost, and outburst floods from glacier-and moraine-dammed lakes will threaten downstream populations.Societies even well beyond the mountains depend on meltwater from glaciers and snow for drinking water supplies, irrigation, mining, hydropower, agriculture, and recreation. Here, we review and, where possible, quantify the impacts of anticipated climate change on the alpine cryosphere, hydrosphere, and biosphere, and consider the implications for adaptation to a future of mountains without permanent snow and ice.}, language = {en} } @misc{SmithBookhagen2018, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Using passive microwave data to understand spatio-temporal trends and dynamics in snow-water storage in High Mountain Asia}, series = {active and passive microwave remote sensing for environmental monitoring II}, volume = {10788}, journal = {active and passive microwave remote sensing for environmental monitoring II}, publisher = {SPIE-INT Soc Optical Engineering}, address = {Bellingham}, isbn = {978-1-5106-2160-2}, issn = {0277-786X}, doi = {10.1117/12.2323827}, pages = {8}, year = {2018}, abstract = {High Mountain Asia provides water for more than a billion downstream users. Many catchments receive the majority of their yearly water budget in the form of snow - the vast majority of which is not monitored by sparse weather networks. We leverage passive microwave data from the SSMI series of satellites (SSMI, SSMI/S, 1987-2016), reprocessed to 3.125 km resolution, to examine trends in the volume and spatial distribution of snow-water equivalent (SWE) in the Indus Basin. We find that the majority of the Indus has seen an increase in snow-water storage. There exists a strong elevation-trend relationship, where high-elevation zones have more positive SWE trends. Negative trends are confined to the Himalayan foreland and deeply-incised valleys which run into the Upper Indus. This implies a temperature-dependent cutoff below which precipitation increases are not translated into increased SWE. Earlier snowmelt or a higher percentage of liquid precipitation could both explain this cutoff.(1) Earlier work 2 found a negative snow-water storage trend for the entire Indus catchment over the time period 1987-2009 (-4 x 10(-3) mm/yr). In this study based on an additional seven years of data, the average trend reverses to 1.4 x 10(-3). This implies that the decade since the mid-2000s was likely wetter, and positively impacted long-term SWE trends. This conclusion is supported by an analysis of snowmelt onset and end dates which found that while long-term trends are negative, more recent (since 2005) trends are positive (moving later in the year).(3)}, language = {en} } @article{CannonCarvalhoJonesetal.2017, author = {Cannon, Forest and Carvalho, Leila M. V. and Jones, Charles and Norris, Jesse and Bookhagen, Bodo and Kiladis, George N.}, title = {Effects of topographic smoothing on the simulation of winter precipitation in High Mountain Asia}, series = {Journal of Geophysical Research: Atmospheres}, volume = {122}, journal = {Journal of Geophysical Research: Atmospheres}, number = {3}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-897X}, doi = {10.1002/2016JD026038}, pages = {1456 -- 1474}, year = {2017}, abstract = {Numerous studies have projected future changes in High Mountain Asia water resources based on temperature and precipitation from global circulation models (GCMs) under future climate scenarios. Although the potential benefit of such studies is immense, coarse grid-scale GCMs are unable to resolve High Mountain Asia's complex topography and thus have a biased representation of regional weather and climate. This study investigates biases in the simulation of physical mechanisms that generate snowfall and contribute to snowpack in High Mountain Asia in coarse topography experiments using the Weather Research and Forecasting model. Regional snowpack is event driven, thus 33 extreme winter orographic precipitation events are simulated at fine atmospheric resolution with 6.67 km resolution topography and smoothed 1.85° × 1.25° GCM topography. As with many modified topography experiments performed in other regions, the distribution of precipitation is highly dependent on first-order orographic effects, which dominate regional meteorology. However, we demonstrate that topographic smoothing enhances circulation in simulated extratropical cyclones, with significant impacts on orographic precipitation. Despite precipitation reductions of 28\% over the highest ranges, due to reduced ascent on windward slopes, total precipitation over the study domain increased by an average of 9\% in smoothed topography experiments on account of intensified extratropical cyclone dynamics and cross-barrier moisture flux. These findings identify an important source of bias in coarse-resolution simulated precipitation in High Mountain Asia, with important implications for the application of GCMs toward projecting future hydroclimate in the region.}, language = {en} } @article{OezcanBookhagenMusaoglu2018, author = {{\"O}zcan, Orkan and Bookhagen, Bodo and Musaoglu, Nebiye}, title = {Impact of the Ataturk Dam Lake on Agro-Meteorological Aspects of the Southeastern Anatolia Region, Turkey}, series = {Journal of the Indian Society of Remote Sensing}, volume = {46}, journal = {Journal of the Indian Society of Remote Sensing}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0255-660X}, doi = {10.1007/s12524-017-0703-9}, pages = {471 -- 481}, year = {2018}, abstract = {In this study, the spatial and temporal impacts of the Ataturk Dam on agro-meteorological aspects of the Southeastern Anatolia region have been investigated. Change detection and environmental impacts due to water-reserve changes in Ataturk Dam Lake have been determined and evaluated using multi-temporal Landsat satellite imageries and meteorological datasets within a period of 1984-2011. These time series have been evaluated for three time periods. Dam construction period constitutes the first part of the study. Land cover/use changes especially on agricultural fields under the Ataturk Dam Lake and its vicinity have been identified between the periods of 1984-1992. The second period comprises the 10-year period after the completion of filling up the reservoir in 1992. At this period, Landsat and meteorological time-series analyses are examined to assess the impact of the Ataturk Dam Lake on selected irrigated agricultural areas. For the last 9-year period from 2002 to 2011, the relationships between seasonal water-reserve changes and irrigated plains under changing climatic factors primarily driving vegetation activity (monthly, seasonal, and annual fluctuations of rainfall rate, air temperature, humidity) on the watershed have been investigated using a 30-year meteorological time series. The results showed that approximately 368 km(2) of agricultural fields have been affected because of inundation due to the Ataturk Dam Lake. However, irrigated agricultural fields have been increased by 56.3\% of the total area (1552 of 2756 km(2)) on Harran Plain within the period of 1984-2011.}, language = {en} } @misc{RheinwaltBookhagen2018, author = {Rheinwalt, Aljoscha and Bookhagen, Bodo}, title = {Network-based flow accumulation for point clouds}, series = {Remote Sensing for Agriculture, Ecosystems, and Hydrology XX}, volume = {10783}, journal = {Remote Sensing for Agriculture, Ecosystems, and Hydrology XX}, publisher = {SPIE-INT Society of Photo-Optical Instrumentation Engineers}, address = {Bellingham}, isbn = {978-1-5106-2150-3}, issn = {0277-786X}, doi = {10.1117/12.2318424}, pages = {12}, year = {2018}, abstract = {Point clouds provide high-resolution topographic data which is often classified into bare-earth, vegetation, and building points and then filtered and aggregated to gridded Digital Elevation Models (DEMs) or Digital Terrain Models (DTMs). Based on these equally-spaced grids flow-accumulation algorithms are applied to describe the hydrologic and geomorphologic mass transport on the surface. In this contribution, we propose a stochastic point-cloud filtering that, together with a spatial bootstrap sampling, allows for a flow accumulation directly on point clouds using Facet-Flow Networks (FFN). Additionally, this provides a framework for the quantification of uncertainties in point-cloud derived metrics such as Specific Catchment Area (SCA) even though the flow accumulation itself is deterministic.}, language = {en} } @article{SmithBookhagen2018, author = {Smith, Taylor and Bookhagen, Bodo}, title = {Changes in seasonal snow water equivalent distribution in High Mountain Asia (1987 to 2009)}, series = {Science Advances}, volume = {4}, journal = {Science Advances}, number = {1}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {2375-2548}, doi = {10.1126/sciadv.1701550}, pages = {8}, year = {2018}, abstract = {Snow meltwaters account for most of the yearly water budgets of many catchments in High Mountain Asia (HMA). We examine trends in snow water equivalent (SWE) using passive microwave data (1987 to 2009). We find an overall decrease in SWE in HMA, despite regions of increased SWE in the Pamir, Kunlun Shan, Eastern Himalaya, and Eastern Tien Shan. Although the average decline in annual SWE across HMA (contributing area, 2641 x 10(3) km(2)) is low (average, -0.3\%), annual SWE losses conceal distinct seasonal and spatial heterogeneities across the study region. For example, the Tien Shan has seen both strong increases in winter SWE and sharp declines in spring and summer SWE. In the majority of catchments, the most negative SWE trends are found in mid-elevation zones, which often correspond to the regions of highest snow-water storage and are somewhat distinct from glaciated areas. Negative changes in SWE storage in these mid-elevation zones have strong implications for downstream water availability.}, language = {en} } @article{WulfBookhagenScherler2012, author = {Wulf, Hendrik and Bookhagen, Bodo and Scherler, Dirk}, title = {Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya}, series = {Hydrology and earth system sciences : HESS}, volume = {16}, journal = {Hydrology and earth system sciences : HESS}, number = {7}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1027-5606}, doi = {10.5194/hess-16-2193-2012}, pages = {2193 -- 2217}, year = {2012}, abstract = {The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001-2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (a parts per thousand yen 99th SSC percentile) coincide frequently (57-80\%) with heavy rainstorms and account for about 30\% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation.}, language = {en} } @article{WulfBookhagenScherler2010, author = {Wulf, Hendrik and Bookhagen, Bodo and Scherler, Dirk}, title = {Seasonal precipitation gradients and their impact on fluvial sediment flux in the Northwest Himalaya}, issn = {0169-555X}, doi = {10.1016/j.geomorph.2009.12.003}, year = {2010}, abstract = {Precipitation in the form of rain and snowfall throughout the Himalaya controls river discharge and erosional processes and, thus, has a first-order control on the fluvial sediment flux. Here, we analyze daily precipitation data (1998-2007) of 80 weather stations from the northwestern Himalaya in order to decipher temporal and spatial moisture gradients. In addition, suspended sediment data allow assessment of the impact of precipitation on the fluvial sediment flux for a 10(3)-km(2) catchment (Baspa). We find that weather stations located at the mountain front receive similar to 80\% of annual precipitation during summer (May-Oct), whereas stations in the orogenic interior, i.e., leeward of the orographic barrier, receive similar to 60\% of annual precipitation during winter (Nov-Apr). In both regions 4-6 rainstorm days account for similar to 40\% of the summer budgets, while rainstorm magnitude-frequency relations, derived from 40-year precipitation time-series, indicate a higher storm variability in the interior than in the frontal region. This high variability in maximum annual rainstorm days in the orogenic interior is reflected by a high variability in extreme suspended sediment events in the Baspa Valley, which strongly affect annual erosion yields. The two most prominent 5-day-long erosional events account for 50\% of the total 5-year suspended sediment flux and coincide with synoptic-scale monsoonal rainstorms. This emphasizes the erosional impact of the Indian Summer Monsoon as the main driving force for erosion processes in the orogenic interior, despite more precipitation falling during the winter season.}, language = {en} } @article{MeeseBookhagenOlenetal.2018, author = {Meese, Bernd and Bookhagen, Bodo and Olen, Stephanie M. and Barthold, Frauke Katrin and Sachse, Dirk}, title = {The effect of Indian Summer Monsoon rainfall on surface water delta D values in the central Himalaya}, series = {Hydrological processes}, volume = {32}, journal = {Hydrological processes}, number = {24}, publisher = {Wiley}, address = {Hoboken}, issn = {0885-6087}, doi = {10.1002/hyp.13281}, pages = {3662 -- 3674}, year = {2018}, abstract = {Stable isotope proxy records, such as speleothems, plant-wax biomarker records, and ice cores, are suitable archives for the reconstruction of regional palaeohydrologic conditions. But the interpretation of these records in the tropics, especially in the Indian Summer Monsoon (ISM) domain, is difficult due to differing moisture and water sources: precipitation from the ISM and Winter Westerlies, as well as snow- and glacial meltwater. In this study, we use interannual differences in ISM strength (2011-2012) to understand the stable isotopic composition of surface water in the Arun River catchment in eastern Nepal. We sampled main stem and tributary water (n = 204) for stable hydrogen and oxygen isotope analysis in the postmonsoon phase of two subsequent years with significantly distinct ISM intensities. In addition to the 2011/2012 sampling campaigns, we collected a 12-month time series of main stem waters (2012/2013, n = 105) in order to better quantify seasonal effects on the variability of surface water delta O-18/delta D. Furthermore, remotely sensed satellite data of rainfall, snow cover, glacial coverage, and evapotranspiration was evaluated. The comparison of datasets from both years revealed that surface waters of the main stem Arun and its tributaries were D-enriched by similar to 15 parts per thousand when ISM rainfall decreased by 20\%. This strong response emphasizes the importance of the ISM for surface water run-off in the central Himalaya. However, further spatio-temporal analysis of remote sensing data in combination with stream water d-excess revealed that most high-altitude tributaries and the Tibetan part of the Arun receive high portions of glacial melt water and likely Winter Westerly Disturbances precipitation. We make the following two implications: First, palaeohydrologic archives found in high-altitude tributaries and on the southern Tibetan Plateau record a mixture of past precipitation delta D values and variable amounts of additional water sources. Second, surface water isotope ratios of lower elevated tributaries strongly reflect the isotopic composition of ISM rainfall implying a suitable region for the analysis of potential delta D value proxy records.}, language = {en} } @article{RegmiBookhagen2022, author = {Regmi, Shakil and Bookhagen, Bodo}, title = {The spatial pattern of extreme precipitation from 40 years of gauge data in the central Himalaya}, series = {Weather and climate extremes}, volume = {37}, journal = {Weather and climate extremes}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2212-0947}, doi = {10.1016/j.wace.2022.100470}, pages = {14}, year = {2022}, abstract = {The topography of the Himalaya exerts a substantial control on the spatial distribution of monsoonal rainfall, which is a vital water source for the regional economy and population. But the occurrence of short-lived and high-intensity precipitation results in socio-economic losses. This study relies on 40 years of daily data from 204 ground stations in Nepal to derive extreme precipitation thresholds, amounts, and days at the 95th percentile. We additionally determine the precipitation magnitude-frequency relation. We observe that extreme precipitation amounts follow an almost uniform band parallel to topographic contour lines in the southern Himalaya mountains in central and eastern Nepal but not in western Nepal. The relationship of extreme precipitation indices with topographic relief shows that extreme precipitation thresholds decrease with increasing elevation, but extreme precipitation days increase in higher elevation areas. Furthermore, stations above 1 km elevation exhibit a power-law relation in the rainfall magnitude-frequency framework. Stations at higher elevations generally have lower values of power-law exponents than low elevation areas. This suggests a fundamentally different behaviour of the rainfall distribution and an increased occurrence of extreme rainfall storms in the high elevation areas of Nepal.}, language = {en} } @article{BergnerTrauthBookhagen2003, author = {Bergner, Andreas G. N. and Trauth, Martin H. and Bookhagen, Bodo}, title = {Magnitude of precipitation : evaporation changes in the Naivasha Basin (Kenya) during the last 150 kyrs}, year = {2003}, abstract = {We modeled the two most extreme highstands of Lake Naivasha during the last 175 k.y. to estimate potential precipitation/ evaporation changes in this basin. In a first step, the bathymetry of the paleolakes at f135 and 9 k.y. BP was reconstructed from sediment cores and surface outcrops. Second, we modeled the paleohydrologic budget during the highstands using a simplified coupled energy mass-balance model. Our results show that the hydrologic and hence the climate conditions at f135 and 9 k.y. BP were similar, but significantly different from today. The main difference is a f15\% higher value in precipitation compared to the present. An adaptation and migration of vegetation in the cause of climate changes would result in a f30\% increase in precipitation. The most likely cause for such a wetter climate at f135 and 9 k.y. BP is a more intense intertropical convergence and increased precipitation in East Africa.}, language = {en} } @article{BookhagenHaseltonTrauth2001, author = {Bookhagen, Bodo and Haselton, Kirk R. and Trauth, Martin H.}, title = {Hydrological modelling of a Pleistocene landslide-dammed lake in the Santa Maria Basin, NW Argentina}, year = {2001}, language = {en} } @inproceedings{ZeilingerMuttiStreckeretal.2006, author = {Zeilinger, Gerold and Mutti, Maria and Strecker, Manfred and Rehak, Katrin and Bookhagen, Bodo and Schwab, Marco}, title = {Integration of digital elevation models and satellite images to investigate geological processes.}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-7063}, year = {2006}, abstract = {In order to better understand the geological boundary conditions for ongoing or past surface processes geologists face two important questions: 1) How can we gain additional knowledge about geological processes by analyzing digital elevation models (DEM) and satellite images and 2) Do these efforts present a viable approach for more efficient research. Here, we will present case studies at a variety of scales and levels of resolution to illustrate how we can substantially complement and enhance classical geological approaches with remote sensing techniques. Commonly, satellite and DEM based studies are being used in a first step of assessing areas of geologic interest. While in the past the analysis of satellite imagery (e.g. Landsat TM) and aerial photographs was carried out to characterize the regional geologic characteristics, particularly structure and lithology, geologists have increasingly ventured into a process-oriented approach. This entails assessing structures and geomorphic features with a concept that includes active tectonics or tectonic activity on time scales relevant to humans. In addition, these efforts involve analyzing and quantifying the processes acting at the surface by integrating different remote sensing and topographic data (e.g. SRTM-DEM, SSM/I, GPS, Landsat 7 ETM, Aster, Ikonos…). A combined structural and geomorphic study in the hyperarid Atacama desert demonstrates the use of satellite and digital elevation data for assessing geological structures formed by long-term (millions of years) feedback mechanisms between erosion and crustal bending (Zeilinger et al., 2005). The medium-term change of landscapes during hundred thousands to millions years in a more humid setting is shown in an example from southern Chile. Based on an analysis of rivers/watersheds combined with landscapes parameterization by using digital elevation models, the geomorphic evolution and change in drainage pattern in the coastal Cordillera can be quantified and put into the context of seismotectonic segmentation of a tectonically active region. This has far-reaching implications for earthquake rupture scenarios and hazard mitigation (K. Rehak, see poster on IMAF Workshop). Two examples illustrate short-term processes on decadal, centennial and millennial time scales: One study uses orogen scale precipitation gradients derived from remotely sensed passive microwave data (Bookhagen et al., 2005a). They demonstrate how debris flows were triggered as a response of slopes to abnormally strong rainfall in the interior parts of the Himalaya during intensified monsoons. The area of the orogen that receives high amounts of precipitation during intensified monsoons also constitutes numerous landslide deposits of up to 1km3 volume that were generated during intensified monsoon phase at about 27 and 9 ka (Bookhagen et al., 2005b). Another project in the Swiss Alps compared sets of aerial photographs recorded in different years. By calculating high resolution surfaces the mass transport in a landslide could be reconstructed (M. Schwab, Universit{\"a}t Bern). All these examples, although representing only a short and limited selection of projects using remote sense data in geology, have as a common approach the goal to quantify geological processes. With increasing data resolution and new sensors future projects will even enable us to recognize more patterns and / or structures indicative of geological processes in tectonically active areas. This is crucial for the analysis of natural hazards like earthquakes, tsunamis and landslides, as well as those hazards that are related to climatic variability. The integration of remotely sensed data at different spatial and temporal scales with field observations becomes increasingly important. Many of presently highly populated places and increasingly utilized regions are subject to significant environmental pressure and often constitute areas of concentrated economic value. Combined remote sensing and ground-truthing in these regions is particularly important as geologic, seismicity and hydrologic data may be limited here due to the recency of infrastructural development. Monitoring ongoing processes and evaluating the remotely sensed data in terms of recurrence of events will greatly enhance our ability to assess and mitigate natural hazards.
Dokument 1: Foliensatz | Dokument 2: Abstract
Interdisziplin{\"a}res Zentrum f{\"u}r Musterdynamik und Angewandte Fernerkundung Workshop vom 9. - 10. Februar 2006}, language = {en} } @misc{RamezaniZiaraniBookhagenSchmidtetal.2019, author = {Ramezani Ziarani, Maryam and Bookhagen, Bodo and Schmidt, Torsten and Wickert, Jens and de la Torre, Alejandro and Hierro, Rodrigo}, title = {Using Convective Available Potential Energy (CAPE) and Dew-Point Temperature to Characterize Rainfall-Extreme Events in the South-Central Andes}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {771}, issn = {1866-8372}, doi = {10.25932/publishup-43886}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-438865}, pages = {22}, year = {2019}, abstract = {The interactions between atmosphere and steep topography in the eastern south-central Andes result in complex relations with inhomogenous rainfall distributions. The atmospheric conditions leading to deep convection and extreme rainfall and their spatial patterns—both at the valley and mountain-belt scales—are not well understood. In this study, we aim to identify the dominant atmospheric conditions and their spatial variability by analyzing the convective available potential energy (CAPE) and dew-point temperature (Td). We explain the crucial effect of temperature on extreme rainfall generation along the steep climatic and topographic gradients in the NW Argentine Andes stretching from the low-elevation eastern foreland to the high-elevation central Andean Plateau in the west. Our analysis relies on version 2.0 of the ECMWF's (European Centre for Medium-RangeWeather Forecasts) Re-Analysis (ERA-interim) data and TRMM (Tropical Rainfall Measuring Mission) data. We make the following key observations: First, we observe distinctive gradients along and across strike of the Andes in dew-point temperature and CAPE that both control rainfall distributions. Second, we identify a nonlinear correlation between rainfall and a combination of dew-point temperature and CAPE through a multivariable regression analysis. The correlation changes in space along the climatic and topographic gradients and helps to explain controlling factors for extreme-rainfall generation. Third, we observe more contribution (or higher importance) of Td in the tropical low-elevation foreland and intermediate-elevation areas as compared to the high-elevation central Andean Plateau for 90th percentile rainfall. In contrast, we observe a higher contribution of CAPE in the intermediate-elevation area between low and high elevation, especially in the transition zone between the tropical and subtropical areas for the 90th percentile rainfall. Fourth, we find that the parameters of the multivariable regression using CAPE and Td can explain rainfall with higher statistical significance for the 90th percentile compared to lower rainfall percentiles. Based on our results, the spatial pattern of rainfall-extreme events during the past ∼16 years can be described by a combination of dew-point temperature and CAPE in the south-central Andes.}, language = {en} } @article{RamezaniZiaraniBookhagenSchmidtetal.2019, author = {Ramezani Ziarani, Maryam and Bookhagen, Bodo and Schmidt, Torsten and Wickert, Jens and de la Torre, Alejandro and Hierro, Rodrigo}, title = {Using Convective Available Potential Energy (CAPE) and Dew-Point Temperature to Characterize Rainfall-Extreme Events in the South-Central Andes}, series = {Atmosphere}, volume = {10}, journal = {Atmosphere}, number = {7}, publisher = {MDPI}, address = {Basel}, issn = {2073-4433}, doi = {10.3390/atmos10070379}, pages = {22}, year = {2019}, abstract = {The interactions between atmosphere and steep topography in the eastern south-central Andes result in complex relations with inhomogenous rainfall distributions. The atmospheric conditions leading to deep convection and extreme rainfall and their spatial patterns—both at the valley and mountain-belt scales—are not well understood. In this study, we aim to identify the dominant atmospheric conditions and their spatial variability by analyzing the convective available potential energy (CAPE) and dew-point temperature (Td). We explain the crucial effect of temperature on extreme rainfall generation along the steep climatic and topographic gradients in the NW Argentine Andes stretching from the low-elevation eastern foreland to the high-elevation central Andean Plateau in the west. Our analysis relies on version 2.0 of the ECMWF's (European Centre for Medium-RangeWeather Forecasts) Re-Analysis (ERA-interim) data and TRMM (Tropical Rainfall Measuring Mission) data. We make the following key observations: First, we observe distinctive gradients along and across strike of the Andes in dew-point temperature and CAPE that both control rainfall distributions. Second, we identify a nonlinear correlation between rainfall and a combination of dew-point temperature and CAPE through a multivariable regression analysis. The correlation changes in space along the climatic and topographic gradients and helps to explain controlling factors for extreme-rainfall generation. Third, we observe more contribution (or higher importance) of Td in the tropical low-elevation foreland and intermediate-elevation areas as compared to the high-elevation central Andean Plateau for 90th percentile rainfall. In contrast, we observe a higher contribution of CAPE in the intermediate-elevation area between low and high elevation, especially in the transition zone between the tropical and subtropical areas for the 90th percentile rainfall. Fourth, we find that the parameters of the multivariable regression using CAPE and Td can explain rainfall with higher statistical significance for the 90th percentile compared to lower rainfall percentiles. Based on our results, the spatial pattern of rainfall-extreme events during the past ∼16 years can be described by a combination of dew-point temperature and CAPE in the south-central Andes.}, language = {en} } @article{RamezaniZiaraniBookhagenSchmidtetal.2021, author = {Ramezani Ziarani, Maryam and Bookhagen, Bodo and Schmidt, Torsten and Wickert, Jens and de la Torre, Alejandro and Deng, Zhiguo and Calori, Andrea}, title = {A model for the relationship between rainfall, GNSS-derived integrated water vapour, and CAPE in the eastern central Andes}, series = {Remote Sensing}, volume = {13}, journal = {Remote Sensing}, number = {18}, publisher = {MDPI}, address = {Basel}, issn = {2072-4292}, doi = {10.3390/rs13183788}, pages = {19}, year = {2021}, abstract = {Atmospheric water vapour content is a key variable that controls the development of deep convective storms and rainfall extremes over the central Andes. Direct measurements of water vapour are challenging; however, recent developments in microwave processing allow the use of phase delays from L-band radar to measure the water vapour content throughout the atmosphere: Global Navigation Satellite System (GNSS)-based integrated water vapour (IWV) monitoring shows promising results to measure vertically integrated water vapour at high temporal resolutions. Previous works also identified convective available potential energy (CAPE) as a key climatic variable for the formation of deep convective storms and rainfall in the central Andes. Our analysis relies on GNSS data from the Argentine Continuous Satellite Monitoring Network, Red Argentina de Monitoreo Satelital Continuo (RAMSAC) network from 1999 to 2013. CAPE is derived from version 2.0 of the ECMWF's (European Centre for Medium-Range Weather Forecasts) Re-Analysis (ERA-interim) and rainfall from the TRMM (Tropical Rainfall Measuring Mission) product. In this study, we first analyse the rainfall characteristics of two GNSS-IWV stations by comparing their complementary cumulative distribution function (CCDF). Second, we separately derive the relation between rainfall vs. CAPE and GNSS-IWV. Based on our distribution fitting analysis, we observe an exponential relation of rainfall to GNSS-IWV. In contrast, we report a power-law relationship between the daily mean value of rainfall and CAPE at the GNSS-IWV station locations in the eastern central Andes that is close to the theoretical relationship based on parcel theory. Third, we generate a joint regression model through a multivariable regression analysis using CAPE and GNSS-IWV to explain the contribution of both variables in the presence of each other to extreme rainfall during the austral summer season. We found that rainfall can be characterised with a higher statistical significance for higher rainfall quantiles, e.g., the 0.9 quantile based on goodness-of-fit criterion for quantile regression. We observed different contributions of CAPE and GNSS-IWV to rainfall for each station for the 0.9 quantile. Fourth, we identify the temporal relation between extreme rainfall (the 90th, 95th, and 99th percentiles) and both GNSS-IWV and CAPE at 6 h time steps. We observed an increase before the rainfall event and at the time of peak rainfall—both for GNSS-integrated water vapour and CAPE. We show higher values of CAPE and GNSS-IWV for higher rainfall percentiles (99th and 95th percentiles) compared to the 90th percentile at a 6-h temporal scale. Based on our correlation analyses and the dynamics of the time series, we show that both GNSS-IWV and CAPE had comparable magnitudes, and we argue to consider both climatic variables when investigating their effect on rainfall extremes.}, language = {en} } @article{AlonsoBookhagenCarrapaetal.2006, author = {Alonso, Ricardo N. and Bookhagen, Bodo and Carrapa, Barbara and Coutand, Isabelle and Haschke, Michael and Hilley, George E. and Schoenbohm, Lindsay M. and Sobel, Edward and Strecker, Manfred and Trauth, Martin H. and Villanueva, Arturo}, title = {Tectonics, climate and landscape evolution of the Southern Central Andes : the Argentine Puna Plateau and adjacent regions between 22 and 30°S}, isbn = {978-3-540- 24329-8}, year = {2006}, language = {en} } @article{StreckerAlonsoBookhagenetal.2009, author = {Strecker, Manfred and Alonso, Ricardo N. and Bookhagen, Bodo and Carrapa, Barbara and Coutand, Isabelle and Hain, Mathis P. and Hilley, George E. and Mortimer, Estelle and Schoenbohm, Lindsay M. and Sobel, Edward}, title = {Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes?}, issn = {0091-7613}, doi = {10.1130/G25545a.1}, year = {2009}, abstract = {Orogenic plateaus are extensive, high-elevation areas with low internal relief that have been attributed to deep-seated and/or climate-driven surface processes. In the latter case, models predict that lateral plateau growth results from increasing aridity along the margins as range uplift shields the orogen interior from precipitation. We analyze the spatiotemporal progression of basin isolation and filling at the eastern margin of the Puna Plateau of the Argentine Andes to determine if the topography predicted by such models is observed. We find that the timing of basin filling and reexcavation is variable, suggesting nonsystematic plateau growth. Instead, the Airy isostatically compensated component of topography constitutes the majority of the mean elevation gain between the foreland and the plateau. This indicates that deep-seated phenomena, such as changes in crustal thickness and/or lateral density, are required to produce high plateau elevations. In contrast, the frequency of the uncompensated topography within the plateau and in the adjacent foreland that is interrupted by ranges appears similar, although the amplitude of this topographic component increases east of the plateau. Combined with sedimentologic observations, we infer that the low internal relief of the plateau likely results from increased aridity and sediment storage within the plateau and along its eastern margin.}, language = {en} } @article{ThiedeArrowsmithBookhagenetal.2006, author = {Thiede, Rasmus Christoph and Arrowsmith, J. Ram{\´o}n and Bookhagen, Bodo and McWilliams, Michael O. and Sobel, Edward and Strecker, Manfred}, title = {Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India}, doi = {10.1130/B25872.1}, year = {2006}, abstract = {Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new field mapping, structural data, and cooling ages from the western flank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post-middle Miocene structural development. The western flank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of high-grade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. Ar- 40/Ar-39 mica and apatite fission track (AFT) ages constrain cooling and exhumation pathways front 350 to 60 degrees C and suggest that the dome cooled in three stages since the middle Miocene. Ar-40/Ar-39 white mica ages of 16-14 Ma suggest a first phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased significantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may reflect more pronounced regionally distributed and erosion-driven exhumation}, language = {en} } @article{ThiedeBookhagenArrowsmithetal.2004, author = {Thiede, Rasmus Christoph and Bookhagen, Bodo and Arrowsmith, J. Ram{\´o}n and Sobel, Edward and Strecker, Manfred}, title = {Climatic control on rapid exhumation along the Southern Himalayan Front}, issn = {0012-821X}, year = {2004}, abstract = {Along the Southern Himalayan Front (SHF), areas with concentrated precipitation coincide with rapid exhumation, as indicated by young mineral cooling ages. Twenty new, young ( < 1-5 Ma) apatite fission track (AFT) ages have been obtained from the Himalayan Crystalline Core along the Sutlej Valley, NW India. The AFT ages correlate with elevation, but show no spatial relationship to tectonic structures, such as the Main Central Thrust or the Southern Tibetan Fault System. Monsoonal precipitation in this region exerts a strong influence on erosional surface processes. Fluvial erosional unloading along the SHF is focused on high mountainous areas, where the orographic barrier forces out > 80\% of the annual precipitation. AFT cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a similar to 50-70-km-wide sector of the Himalaya, rather than encompassing the entire orogen. Assuming simplified constant exhumation rates, the rocks of two age vs. elevation transects were exhumed at similar to 1.4 +/- 0.2 and similar to 1.1 +/- 0.4 mm/a with an average cooling rate of similar to 40-50degreesC/Ma during Pliocene-Quarternary time. Following other recently published hypotheses regarding the relation between tectonics and climate in the Himalaya, we suggest that this concentrated loss of material was accommodated by motion along a back-stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge. Climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the Himalaya. (C) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{ThiedeArrowsmithBookhagenetal.2005, author = {Thiede, Rasmus Christoph and Arrowsmith, J. Ram{\´o}n and Bookhagen, Bodo and McWilliams, Michael O. and Sobel, Edward and Strecker, Manfred}, title = {From tectonically to erosionally controlled development of the Himalayan orogen}, issn = {0091-7613}, year = {2005}, abstract = {Whether variations in the spatial distribution of erosion influence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new Ar-40/Ar-39 white mica and apatite fission- track (AFT) ages that measure the vertical component of exhumation rates along an similar to 120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The Ar-40/Ar-39 data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crystalline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented similar to 80 x 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation defined by the AFT data correlates with a region of high precipitation, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene}, language = {en} } @misc{MilewskiChabrillatBookhagen2020, author = {Milewski, Robert and Chabrillat, Sabine and Bookhagen, Bodo}, title = {Analyses of Namibian Seasonal Salt Pan Crust Dynamics and Climatic Drivers Using Landsat 8 Time-Series and Ground Data}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {988}, issn = {1866-8372}, doi = {10.25932/publishup-47568}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-475685}, pages = {26}, year = {2020}, abstract = {Salt pans are highly dynamic environments that are difficult to study by in situ methods because of their harsh climatic conditions and large spatial areas. Remote sensing can help to elucidate their environmental dynamics and provide important constraints regarding their sedimentological, mineralogical, and hydrological evolution. This study utilizes spaceborne multitemporal multispectral optical data combined with spectral endmembers to document spatial distribution of surface crust types over time on the Omongwa pan located in the Namibian Kalahari. For this purpose, 49 surface samples were collected for spectral and mineralogical characterization during three field campaigns (2014-2016) reflecting different seasons and surface conditions of the salt pan. An approach was developed to allow the spatiotemporal analysis of the salt pan crust dynamics in a dense time-series consisting of 77 Landsat 8 cloud-free scenes between 2014 and 2017, covering at least three major wet-dry cycles. The established spectral analysis technique Sequential Maximum Angle Convex Cone (SMACC) extraction method was used to derive image endmembers from the Landsat time-series stack. Evaluation of the extracted endmember set revealed that the multispectral data allowed the differentiation of four endmembers associated with mineralogical mixtures of the crust's composition in dry conditions and three endmembers associated with flooded or muddy pan conditions. The dry crust endmember spectra have been identified in relation to visible, near infrared, and short-wave infrared (VNIR-SWIR) spectroscopy and X-ray diffraction (XRD) analyses of the collected surface samples. According these results, the spectral endmembers are interpreted as efflorescent halite crust, mixed halite-gypsum crust, mixed calcite quartz sepiolite crust, and gypsum crust. For each Landsat scene the spatial distribution of these crust types was mapped with the Spectral Angle Mapper (SAM) method and significant spatiotemporal dynamics of the major surface crust types were observed. Further, the surface crust dynamics were analyzed in comparison with the pan's moisture regime and other climatic parameters. The results show that the crust dynamics are mainly driven by flooding events in the wet season, but are also influenced by temperature and aeolian activity in the dry season. The approach utilized in this study combines the advantages of multitemporal satellite data for temporal event characterization with advantages from hyperspectral methods for the image and ground data analyses that allow improved mineralogical differentiation and characterization.}, language = {en} } @misc{RamezaniZiaraniBookhagenSchmidtetal.2021, author = {Ramezani Ziarani, Maryam and Bookhagen, Bodo and Schmidt, Torsten and Wickert, Jens and de la Torre, Alejandro and Deng, Zhiguo and Calori, Andrea}, title = {A model for the relationship between rainfall, GNSS-derived integrated water vapour, and CAPE in the eastern central Andes}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1172}, issn = {1866-8372}, doi = {10.25932/publishup-52325}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-523256}, pages = {21}, year = {2021}, abstract = {Atmospheric water vapour content is a key variable that controls the development of deep convective storms and rainfall extremes over the central Andes. Direct measurements of water vapour are challenging; however, recent developments in microwave processing allow the use of phase delays from L-band radar to measure the water vapour content throughout the atmosphere: Global Navigation Satellite System (GNSS)-based integrated water vapour (IWV) monitoring shows promising results to measure vertically integrated water vapour at high temporal resolutions. Previous works also identified convective available potential energy (CAPE) as a key climatic variable for the formation of deep convective storms and rainfall in the central Andes. Our analysis relies on GNSS data from the Argentine Continuous Satellite Monitoring Network, Red Argentina de Monitoreo Satelital Continuo (RAMSAC) network from 1999 to 2013. CAPE is derived from version 2.0 of the ECMWF's (European Centre for Medium-Range Weather Forecasts) Re-Analysis (ERA-interim) and rainfall from the TRMM (Tropical Rainfall Measuring Mission) product. In this study, we first analyse the rainfall characteristics of two GNSS-IWV stations by comparing their complementary cumulative distribution function (CCDF). Second, we separately derive the relation between rainfall vs. CAPE and GNSS-IWV. Based on our distribution fitting analysis, we observe an exponential relation of rainfall to GNSS-IWV. In contrast, we report a power-law relationship between the daily mean value of rainfall and CAPE at the GNSS-IWV station locations in the eastern central Andes that is close to the theoretical relationship based on parcel theory. Third, we generate a joint regression model through a multivariable regression analysis using CAPE and GNSS-IWV to explain the contribution of both variables in the presence of each other to extreme rainfall during the austral summer season. We found that rainfall can be characterised with a higher statistical significance for higher rainfall quantiles, e.g., the 0.9 quantile based on goodness-of-fit criterion for quantile regression. We observed different contributions of CAPE and GNSS-IWV to rainfall for each station for the 0.9 quantile. Fourth, we identify the temporal relation between extreme rainfall (the 90th, 95th, and 99th percentiles) and both GNSS-IWV and CAPE at 6 h time steps. We observed an increase before the rainfall event and at the time of peak rainfall—both for GNSS-integrated water vapour and CAPE. We show higher values of CAPE and GNSS-IWV for higher rainfall percentiles (99th and 95th percentiles) compared to the 90th percentile at a 6-h temporal scale. Based on our correlation analyses and the dynamics of the time series, we show that both GNSS-IWV and CAPE had comparable magnitudes, and we argue to consider both climatic variables when investigating their effect on rainfall extremes.}, language = {en} } @article{ThompsonChenYangetal.2018, author = {Thompson, Jessica A. and Chen, Jie and Yang, Huili and Li, Tao and Bookhagen, Bodo and Burbank, Douglas}, title = {Coarse- versus fine-grain quartz OSL and cosmogenic Be-10 dating of deformed fluvial terraces on the northeast Pamir margin, northwest China}, series = {Quaternary geochronology : the international research and review journal on advances in quaternary dating techniques}, volume = {46}, journal = {Quaternary geochronology : the international research and review journal on advances in quaternary dating techniques}, publisher = {Elsevier}, address = {Oxford}, issn = {1871-1014}, doi = {10.1016/j.quageo.2018.01.002}, pages = {1 -- 15}, year = {2018}, abstract = {Along the NE Pamir margin, flights of late Quaternary fluvial terraces span actively deforming fault-related folds. We present detailed results on two terraces dated using optically stimulated luminescence (OSL) and cosmogenic radionuclide Be-10 (CRN) techniques. Quartz OSL dating of two different grain sizes (4-11 mu m and 90-180 mu m) revealed the fine-grain quartz fraction may overestimate the terrace ages by up to a factor of ten. Two-mm, small-aliquot, coarse-grain quartz OSL ages, calculated using the minimum age model, yielded stratigraphically consistent ages within error and dated times of terrace deposition to similar to 9 and similar to 16 ka. We speculate that, in this arid environment, fine-grain samples can be transported and deposited in single, turbid, and (sometimes) night-time floods that prevent thorough bleaching and, thereby, can lead to relatively large residual OSL signals. In contrast, sand in the fluvial system is likely to have a much longer residence time during transport, thereby providing greater opportunities for thorough bleaching. CRN Be-10 depth profiles date the timing of terrace abandonment to similar to 8 and similar to 14 ka: ages that generally agree with the coarse-grain quartz OSL ages. Our new terrace age of similar to 13-14 ka is broadly consistent with other terraces in the region that indicate terrace deposition and subsequent abandonment occurred primarily during glacial-interglacial transitions, thereby suggesting a climatic control on the formation of these terraces on the margins of the Tarim Basin. Furthermore, tectonic shortening rates calculated from these deformed terraces range from similar to 1.2 to similar to 4.6 mm/a and, when combined with shortening rates from other structures in the region, illuminate the late Quaternary basinward migration of deformation to faults and folds along the Pamir-Tian Shan collisional interface.}, language = {en} } @article{NennewitzThiedeBookhagen2018, author = {Nennewitz, Markus and Thiede, Rasmus C. and Bookhagen, Bodo}, title = {Fault activity, tectonic segmentation, and deformation pattern of the western Himalaya on Ma timescales inferred from landscape morphology}, series = {Lithosphere}, volume = {10}, journal = {Lithosphere}, number = {5}, publisher = {American Institute of Physics}, address = {Boulder}, issn = {1941-8264}, doi = {10.1130/L681.1}, pages = {632 -- 640}, year = {2018}, abstract = {The location and magnitude of Himalayan tectonic activity has been debated for decades, and several aspects remain unknown. For instance, the spatial distribution of crustal shortening that ultimately sustains Himalayan topography and the activity of major fault zones remain unknown at Ma timescales. In this study, we address the spatial deformation pattern in the data-scarce western Himalaya. We calculated catchment averaged, normalized river-steepness indices of non-glaciated drainage basins with tributary catchment areas between 5 and 200 km(2) (n = 2138). We analyzed the spatial distribution of the relative change of river steepness both along and across strike to gain information about the regional distribution of differential uplift pattern and relate this to the activity of distinctive fault segments. For our study area, we observe a positive correlation of averaged k(sn) values with long-term exhumation rates derived from previously published thermochronologic datasets combined with thermal modeling as well as with millennial timescale denudation rates based on cosmogenic nuclide dating. Our results indicate three tectono-geomorphic segments with distinctive landscape morphology, structural architecture, and fault geometry along the western Himalaya: Garhwal-Sutlej, Chamba, and Kashmir Himalaya (from east to west). Moreover, our data recognize distinctive fault segments showing varying thrust activity along strike of the Main Frontal Thrust, the Main Boundary Thrust, and in the vicinity of the steep topographic transition between the Lesser and Greater Himalaya. In this region, we relate out-of-sequence deformation along major basement thrust ramps, such as the Munsiari Thrust with deformation along a mid-crustal ramp along the basal decollement. We suggest that during the Quaternary, all major fault zones in the Western Himalaya experienced out-of-sequence faulting and have accommodated some portion of crustal shortening.}, language = {en} } @article{GrujicGovinBarrieretal.2018, author = {Grujic, Djordje and Govin, Gwladys and Barrier, Laurie and Bookhagen, Bodo and Coutand, Isabelle and Cowan, Beth and Hren, Michael T. and Najman, Yani}, title = {Formation of a Rain Shadow}, series = {Geochemistry, geophysics, geosystems}, volume = {19}, journal = {Geochemistry, geophysics, geosystems}, number = {9}, publisher = {American Geophysical Union}, address = {Washington}, issn = {1525-2027}, doi = {10.1029/2017GC007254}, pages = {3430 -- 3447}, year = {2018}, abstract = {We measure the oxygen and hydrogen stable isotope composition of authigenic clays from Himalayan foreland sediments (Siwalik Group), and from present day small stream waters in eastern Bhutan to explore the impact of uplift of the Shillong Plateau on rain shadow formation over the Himalayan foothills. Stable isotope data from authigenic clay minerals (<2 μm) suggest the presence of three paleoclimatic periods during deposition of the Siwalik Group, between ∼7 and ∼1 Ma. The mean δ18O value in paleometeoric waters, which were in equilibrium with clay minerals, is ∼2.5 per mille lower than in modern meteoric and stream waters at the elevation of the foreland basin. We discuss the factors that could have changed the isotopic composition of water over time and we conclude that (a) the most likely and significant cause for the increase in meteoric water δ18O values over time is the "amount effect," specifically, a decrease in mean annual precipitation. (b) The change in mean annual precipitation over the foreland basin and foothills of the Himalaya is the result of orographic effect caused by the Shillong Plateau's uplift. The critical elevation of the Shillong Plateau required to induce significant orographic precipitation was attained after ∼1.2 Ma. (c) By applying scale analysis, we estimate that the mean annual precipitation over the foreland basin of the eastern Bhutan Himalayas has decreased by a factor of 1.7-2.5 over the last 1-3 million years.}, language = {en} } @article{BrellSeglGuanteretal.2019, author = {Brell, Maximilian and Segl, Karl and Guanter, Luis and Bookhagen, Bodo}, title = {3D hyperspectral point cloud generation}, series = {ISPRS journal of photogrammetry and remote sensing : official publication of the International Society for Photogrammetry and Remote Sensing}, volume = {149}, journal = {ISPRS journal of photogrammetry and remote sensing : official publication of the International Society for Photogrammetry and Remote Sensing}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0924-2716}, doi = {10.1016/j.isprsjprs.2019.01.022}, pages = {200 -- 214}, year = {2019}, abstract = {Remote Sensing technologies allow to map biophysical, biochemical, and earth surface parameters of the land surface. Of especial interest for various applications in environmental and urban sciences is the combination of spectral and 3D elevation information. However, those two data streams are provided separately by different instruments, namely airborne laser scanner (ALS) for elevation and a hyperspectral imager (HSI) for high spectral resolution data. The fusion of ALS and HSI data can thus lead to a single data entity consistently featuring rich structural and spectral information. In this study, we present the application of fusing the first pulse return information from ALS data at a sub-decimeter spatial resolution with the lower-spatial resolution hyperspectral information available from the HSI into a hyperspectral point cloud (HSPC). During the processing, a plausible hyperspectral spectrum is assigned to every first-return ALS point. We show that the complementary implementation of spectral and 3D information at the point-cloud scale improves object-based classification and information extraction schemes. This improvements have great potential for numerous land cover mapping and environmental applications.}, language = {en} } @article{JobeLiBookhagenetal.2018, author = {Jobe, Jessica Ann Thompson and Li, Tao and Bookhagen, Bodo and Chen, Jie and Burbank, Douglas W.}, title = {Dating growth strata and basin fill by combining Al-26/Be-10 burial dating and magnetostratigraphy}, series = {Lithosphere}, volume = {10}, journal = {Lithosphere}, number = {6}, publisher = {American Institute of Physics}, address = {Boulder}, issn = {1941-8264}, doi = {10.1130/L727.1}, pages = {806 -- 828}, year = {2018}, abstract = {Cosmogenic burial dating enables dating of coarse-grained, Pliocene-Pleistocene sedimentary units that are typically difficult to date with traditional methods, such as magnetostratigraphy. In the actively deforming western Tarim Basin in NW China, Pliocene-Pleistocene conglomerates were dated at eight sites, integrating Al-26/Be-10 burial dating with previously published magnetostratigraphic sections. These samples were collected from growth strata on the flanks of growing folds and from sedimentary units beneath active faults to place timing constraints on the initiation of deformation of structures within the basin and on shortening rates on active faults. These new basin-fill and growthstrata ages document the late Neogene and Quaternary growth of the Pamir and Tian Shan orogens between >5 and 1 Ma and delineate the eastward propagation of deformation at rates up to 115 km/m.y. and basinward growth of both mountain belts at rates up to 12 km/m.y.}, language = {en} } @misc{SmithRheinwaltBookhagen2019, author = {Smith, Taylor and Rheinwalt, Aljoscha and Bookhagen, Bodo}, title = {Determining the optimal grid resolution for topographic analysis on an airborne lidar dataset}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {725}, issn = {1866-8372}, doi = {10.25932/publishup-43016}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-430165}, pages = {475 -- 489}, year = {2019}, abstract = {Digital elevation models (DEMs) are a gridded representation of the surface of the Earth and typically contain uncertainties due to data collection and processing. Slope and aspect estimates on a DEM contain errors and uncertainties inherited from the representation of a continuous surface as a grid (referred to as truncation error; TE) and from any DEM uncertainty. We analyze in detail the impacts of TE and propagated elevation uncertainty (PEU) on slope and aspect. Using synthetic data as a control, we define functions to quantify both TE and PEU for arbitrary grids. We then develop a quality metric which captures the combined impact of both TE and PEU on the calculation of topographic metrics. Our quality metric allows us to examine the spatial patterns of error and uncertainty in topographic metrics and to compare calculations on DEMs of different sizes and accuracies. Using lidar data with point density of ∼10 pts m-2 covering Santa Cruz Island in southern California, we are able to generate DEMs and uncertainty estimates at several grid resolutions. Slope (aspect) errors on the 1 m dataset are on average 0.3∘ (0.9∘) from TE and 5.5∘ (14.5∘) from PEU. We calculate an optimal DEM resolution for our SCI lidar dataset of 4 m that minimizes the error bounds on topographic metric calculations due to the combined influence of TE and PEU for both slope and aspect calculations over the entire SCI. Average slope (aspect) errors from the 4 m DEM are 0.25∘ (0.75∘) from TE and 5∘ (12.5∘) from PEU. While the smallest grid resolution possible from the high-density SCI lidar is not necessarily optimal for calculating topographic metrics, high point-density data are essential for measuring DEM uncertainty across a range of resolutions.}, language = {en} } @article{SmithBookhagenCannon2015, author = {Smith, Taylor and Bookhagen, Bodo and Cannon, Forest}, title = {Improving semi-automated glacier mapping with a multi-method approach: applications in central Asia}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {9}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, number = {5}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1994-0416}, doi = {10.5194/tc-9-1747-2015}, pages = {1747 -- 1759}, year = {2015}, abstract = {Studies of glaciers generally require precise glacier outlines. Where these are not available, extensive manual digitization in a geographic information system (GIS) must be performed, as current algorithms struggle to delineate glacier areas with debris cover or other irregular spectral profiles. Although several approaches have improved upon spectral band ratio delineation of glacier areas, none have entered wide use due to complexity or computational intensity. In this study, we present and apply a glacier mapping algorithm in Central Asia which delineates both clean glacier ice and debris-covered glacier tongues. The algorithm is built around the unique velocity and topographic characteristics of glaciers and further leverages spectral and spatial relationship data. We found that the algorithm misclassifies between 2 and 10\% of glacier areas, as compared to a similar to 750 glacier control data set, and can reliably classify a given Landsat scene in 3-5 min. The algorithm does not completely solve the difficulties inherent in classifying glacier areas from remotely sensed imagery but does represent a significant improvement over purely spectral-based classification schemes, such as the band ratio of Landsat 7 bands three and five or the normalized difference snow index. The main caveats of the algorithm are (1) classification errors at an individual glacier level, (2) reliance on manual intervention to separate connected glacier areas, and (3) dependence on fidelity of the input Landsat data.}, language = {en} } @article{ThompsonBurbankLietal.2015, author = {Thompson, Jessica A. and Burbank, Douglas W. and Li, Tao and Chen, Jie and Bookhagen, Bodo}, title = {Late Miocene northward propagation of the northeast Pamir thrust system, northwest China}, series = {Tectonics}, volume = {34}, journal = {Tectonics}, number = {3}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0278-7407}, doi = {10.1002/2014TC003690}, pages = {510 -- 534}, year = {2015}, abstract = {Piggyback basins on the margins of growing orogens commonly serve as sensitive recorders of the onset of thrust deformation and changes in source areas. The Bieertuokuoyi piggyback basin, located in the hanging wall of the Pamir Frontal Thrust, provides an unambiguous record of the outward growth of the northeast Pamir margin in northwest China from the Miocene through the Quaternary. To reconstruct the deformation along the margin, we synthesized structural mapping, stratigraphy, magnetostratigraphy, and cosmogenic burial dating of basin fill and growth strata. The Bieertuokuoyi basin records the initiation of the Pamir Frontal Thrust and the Takegai Thrust similar to 5-6Ma, as well as clast provenance and paleocurrent changes resulting from the Pliocene-to-Recent uplift and exhumation of the Pamir to the south. Our results show that coeval deformation was accommodated on the major structures on the northeast Pamir margin throughout the Miocene to Recent. Furthermore, our data support a change in the regional kinematics around the Miocene-Pliocene boundary (similar to 5-6Ma). Rapid exhumation of NE Pamir extensional domes, coupled with cessation of the Kashgar-Yecheng Transfer System on the eastern margin of the Pamir, accelerated the outward propagation of the northeastern Pamir margin and the southward propagation of the Kashi-Atushi fold-and-thrust belt in the southern Tian Shan. This coeval deformation signifies the coupling of the Pamir and Tarim blocks and the transfer of shortening north to the Pamir frontal faults and across the quasi-rigid Tarim Basin to the southern Tian Shan Kashi-Atushi fold-and-thrust system.}, language = {en} } @article{HarveyBurbankBookhagen2015, author = {Harvey, Jonathan E. and Burbank, Douglas W. and Bookhagen, Bodo}, title = {Along-strike changes in Himalayan thrust geometry: Topographic and tectonic discontinuities in western Nepal}, series = {Lithosphere}, volume = {7}, journal = {Lithosphere}, number = {5}, publisher = {American Institute of Physics}, address = {Boulder}, issn = {1941-8264}, doi = {10.1130/L444.1}, pages = {511 -- 518}, year = {2015}, abstract = {Geodetic and seismologic studies support a tectonic model for the central Himalaya wherein similar to 2 cm/yr of Indo-Asian convergence is accommodated along the primary decollement under the range, the Main Himalayan thrust. A steeper midcrustal ramp in the Main Himalayan thrust is commonly invoked as driving rapid rock uplift along a range-parallel band in the Greater Himalaya. This tectonic model, developed primarily from studies in central Nepal, is commonly assumed to project along strike with little lateral variation in Main Himalayan thrust geometry or associated rock uplift patterns. Here, we synthesize multiple lines of evidence for a major discontinuity in the Main Himalayan thrust in western Nepal. Analysis of topography and seismicity indicates that west of similar to 82.5 degrees E, the single band of steep topography and seismicity along the Main Himalayan thrust ramp in central Nepal bifurcates around a high-elevation, low-relief landscape, resulting in a two-step topographic front along an similar to 150 km segment of the central Himalaya. Although multiple models could explain this bifurcation, the full suite of data appears to be most consistent with a northward bend to the Main Himalayan thrust ramp and activation of a young duplex horse to the south. This poorly documented segmentation of the Main Himalayan thrust has important implications for the seismogenic potential of the western Nepal seismic gap and for models of the ongoing evolution of the orogen.}, language = {en} } @article{PurintonBookhagen2019, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Introducing PebbleCounts}, series = {Earth Surface Dynamics}, volume = {2019}, journal = {Earth Surface Dynamics}, number = {7}, publisher = {Copernicus Publ}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-7-859-2019}, pages = {859 -- 877}, year = {2019}, abstract = {Grain-size distributions are a key geomorphic metric of gravel-bed rivers. Traditional measurement methods include manual counting or photo sieving, but these are achievable only at the 1-10 ㎡ scale. With the advent of drones and increasingly high-resolution cameras, we can now generate orthoimagery over hectares at millimeter to centimeter resolution. These scales, along with the complexity of high-mountain rivers, necessitate different approaches for photo sieving. As opposed to other image segmentation methods that use a watershed approach, our open-source algorithm, PebbleCounts, relies on k-means clustering in the spatial and spectral domain and rapid manual selection of well-delineated grains. This improves grain-size estimates for complex riverbed imagery, without post-processing. We also develop a fully automated method, PebbleCountsAuto, that relies on edge detection and filtering suspect grains, without the k-means clustering or manual selection steps. The algorithms are tested in controlled indoor conditions on three arrays of pebbles and then applied to 12 × 1 ㎡ orthomosaic clips of high-energy mountain rivers collected with a camera-on-mast setup (akin to a low-flying drone). A 20-pixel b-axis length lower truncation is necessary for attaining accurate grain-size distributions. For the k-means PebbleCounts approach, average percentile bias and precision are 0.03 and 0.09 ψ, respectively, for ∼1.16 mm pixel⁻¹ images, and 0.07 and 0.05 ψ for one 0.32 mm pixel⁻¹ image. The automatic approach has higher bias and precision of 0.13 and 0.15 ψ, respectively, for ∼1.16 mm pixel⁻¹ images, but similar values of -0.06 and 0.05 ψ for one 0.32 mm pixel⁻¹ image. For the automatic approach, only at best 70 \% of the grains are correct identifications, and typically around 50 \%. PebbleCounts operates most effectively at the 1 ㎡ patch scale, where it can be applied in ∼5-10 min on many patches to acquire accurate grain-size data over 10-100 ㎡ areas. These data can be used to validate PebbleCountsAuto, which may be applied at the scale of entire survey sites (102-104 ㎡ ). We synthesize results and recommend best practices for image collection, orthomosaic generation, and grain-size measurement using both algorithms.}, language = {en} } @article{HartmanBookhagenChadwick2016, author = {Hartman, Brett D. and Bookhagen, Bodo and Chadwick, Oliver A.}, title = {The effects of check dams and other erosion control structures on the restoration of Andean bofedal ecosystems}, series = {Restoration Ecology}, volume = {24}, journal = {Restoration Ecology}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {1061-2971}, doi = {10.1111/rec.12402}, pages = {761 -- 772}, year = {2016}, abstract = {Restoring degraded lands in rural environments that are heavily managed to meet subsistence needs is a challenge due to high rates of disturbance and resource extraction. This study investigates the efficacy of erosion control structures (ECSs) as restoration tools in the context of a watershed rehabilitation and wet meadow (bofedal) restoration program in the Bolivian Andes. In an effort to enhance water security and increase grazing stability, Aymara indigenous communities built over 15,000 check dams, 9,100 terraces, 5,300 infiltration ditches, and 35 pasture improvement trials. Communities built ECSs at different rates, and we compared vegetation change in the highest restoration management intensity, lowest restoration management intensity, and nonproject control communities. We used line transects to measure changes in vegetation cover and standing water in gullies with check dams and without check dams, and related these ground measurements to a time series (1986-2009) of normalized difference vegetation index derived from Landsat TM5 images. Evidence suggests that check dams increase bofedal vegetation and standing water at a local scale, and lead to increased greenness at a basin scale when combined with other ECSs. Watershed rehabilitation enhances ecosystem services significant to local communities (grazing stability, water security), which creates important synergies when conducting land restoration in rural development settings.}, language = {en} } @article{SmithRheinwaltBookhagen2019, author = {Smith, Taylor and Rheinwalt, Aljoscha and Bookhagen, Bodo}, title = {Determining the optimal grid resolution for topographic analysis on an airborne lidar dataset}, series = {Earth Surface Dynamics}, volume = {7}, journal = {Earth Surface Dynamics}, publisher = {Copernicus Publ.}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-7-475-2019}, pages = {475 -- 489}, year = {2019}, abstract = {Digital elevation models (DEMs) are a gridded representation of the surface of the Earth and typically contain uncertainties due to data collection and processing. Slope and aspect estimates on a DEM contain errors and uncertainties inherited from the representation of a continuous surface as a grid (referred to as truncation error; TE) and from any DEM uncertainty. We analyze in detail the impacts of TE and propagated elevation uncertainty (PEU) on slope and aspect. Using synthetic data as a control, we define functions to quantify both TE and PEU for arbitrary grids. We then develop a quality metric which captures the combined impact of both TE and PEU on the calculation of topographic metrics. Our quality metric allows us to examine the spatial patterns of error and uncertainty in topographic metrics and to compare calculations on DEMs of different sizes and accuracies. Using lidar data with point density of ∼10 pts m-2 covering Santa Cruz Island in southern California, we are able to generate DEMs and uncertainty estimates at several grid resolutions. Slope (aspect) errors on the 1 m dataset are on average 0.3∘ (0.9∘) from TE and 5.5∘ (14.5∘) from PEU. We calculate an optimal DEM resolution for our SCI lidar dataset of 4 m that minimizes the error bounds on topographic metric calculations due to the combined influence of TE and PEU for both slope and aspect calculations over the entire SCI. Average slope (aspect) errors from the 4 m DEM are 0.25∘ (0.75∘) from TE and 5∘ (12.5∘) from PEU. While the smallest grid resolution possible from the high-density SCI lidar is not necessarily optimal for calculating topographic metrics, high point-density data are essential for measuring DEM uncertainty across a range of resolutions.}, language = {en} } @article{HoffmannFeakinsBookhagenetal.2016, author = {Hoffmann, Bernd and Feakins, Sarah J. and Bookhagen, Bodo and Olen, Stephanie M. and Adhikari, Danda P. and Mainali, Janardan and Sachse, Dirk}, title = {Climatic and geomorphic drivers of plant organic matter transport in the Arun River, E Nepal}, series = {Earth \& planetary science letters}, volume = {452}, journal = {Earth \& planetary science letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-821X}, doi = {10.1016/j.epsl.2016.07.008}, pages = {104 -- 114}, year = {2016}, language = {en} } @misc{JobeLiBookhagenetal.2018, author = {Jobe, Jessica Ann Thompson and Li, Tao and Bookhagen, Bodo and Chen, Jie and Burbank, Douglas W.}, title = {Dating growth strata and basin fill by combining 26Al/10Be burial dating and magnetostratigraphy}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1044}, issn = {1866-8372}, doi = {10.25932/publishup-46806}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-468067}, pages = {806 -- 828}, year = {2018}, abstract = {Cosmogenic burial dating enables dating of coarse-grained, Pliocene-Pleistocene sedimentary units that are typically difficult to date with traditional methods, such as magnetostratigraphy. In the actively deforming western Tarim Basin in NW China, Pliocene-Pleistocene conglomerates were dated at eight sites, integrating Al-26/Be-10 burial dating with previously published magnetostratigraphic sections. These samples were collected from growth strata on the flanks of growing folds and from sedimentary units beneath active faults to place timing constraints on the initiation of deformation of structures within the basin and on shortening rates on active faults. These new basin-fill and growthstrata ages document the late Neogene and Quaternary growth of the Pamir and Tian Shan orogens between >5 and 1 Ma and delineate the eastward propagation of deformation at rates up to 115 km/m.y. and basinward growth of both mountain belts at rates up to 12 km/m.y.}, language = {en} } @misc{PurintonBookhagen2021, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Beyond Vertical Point Accuracy}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, publisher = {Universit{\"a}tsverlag Potsdam}, address = {Potsdam}, issn = {1866-8372}, doi = {10.25932/publishup-54980}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-549805}, pages = {1 -- 24}, year = {2021}, abstract = {Quantitative geomorphic research depends on accurate topographic data often collected via remote sensing. Lidar, and photogrammetric methods like structure-from-motion, provide the highest quality data for generating digital elevation models (DEMs). Unfortunately, these data are restricted to relatively small areas, and may be expensive or time-consuming to collect. Global and near-global DEMs with 1 arcsec (∼30 m) ground sampling from spaceborne radar and optical sensors offer an alternative gridded, continuous surface at the cost of resolution and accuracy. Accuracy is typically defined with respect to external datasets, often, but not always, in the form of point or profile measurements from sources like differential Global Navigation Satellite System (GNSS), spaceborne lidar (e.g., ICESat), and other geodetic measurements. Vertical point or profile accuracy metrics can miss the pixel-to-pixel variability (sometimes called DEM noise) that is unrelated to true topographic signal, but rather sensor-, orbital-, and/or processing-related artifacts. This is most concerning in selecting a DEM for geomorphic analysis, as this variability can affect derivatives of elevation (e.g., slope and curvature) and impact flow routing. We use (near) global DEMs at 1 arcsec resolution (SRTM, ASTER, ALOS, TanDEM-X, and the recently released Copernicus) and develop new internal accuracy metrics to assess inter-pixel variability without reference data. Our study area is in the arid, steep Central Andes, and is nearly vegetation-free, creating ideal conditions for remote sensing of the bare-earth surface. We use a novel hillshade-filtering approach to detrend long-wavelength topographic signals and accentuate short-wavelength variability. Fourier transformations of the spatial signal to the frequency domain allows us to quantify: 1) artifacts in the un-projected 1 arcsec DEMs at wavelengths greater than the Nyquist (twice the nominal resolution, so > 2 arcsec); and 2) the relative variance of adjacent pixels in DEMs resampled to 30-m resolution (UTM projected). We translate results into their impact on hillslope and channel slope calculations, and we highlight the quality of the five DEMs. We find that the Copernicus DEM, which is based on a carefully edited commercial version of the TanDEM-X, provides the highest quality landscape representation, and should become the preferred DEM for topographic analysis in areas without sufficient coverage of higher-quality local DEMs.}, language = {en} } @article{ErbelloDoelessoMelnickZeilingeretal.2022, author = {Erbello Doelesso, Asfaw and Melnick, Daniel and Zeilinger, Gerold and Bookhagen, Bodo and Pingel, Heiko and Strecker, Manfred}, title = {Geomorphic expression of a tectonically active rift-transfer zone in southern Ethiopia}, series = {Geomorphology : an international journal on pure and applied geomorphology}, volume = {403}, journal = {Geomorphology : an international journal on pure and applied geomorphology}, publisher = {Elsevier Science}, address = {Amsterdam [u.a.]}, issn = {0169-555X}, doi = {10.1016/j.geomorph.2022.108162}, pages = {20}, year = {2022}, abstract = {The Gofa Province and the Chew Bahir Basin of southern Ethiopia constitute tectonically active regions, where the Southern Main Ethiopian Rift converges with the Northern Kenya Rift through a wide zone of extensional deformation with several north to northeast-trending, left-stepping en-e \& PRIME;chelon basins. This sector of the Southern Main Ethiopian Rift is characterized by a semi-arid climate and a largely uniform lithology, and thus provides ideal conditions for studying the different parameters that define the tectonic and geomorphic features of this complex kinematic transfer zone. In this study, the degree of tectonic activity, spatiotemporal variations in extension, and the nature of kinematic linkage between different fault systems of the transfer zone are constrained by detailed quantitative geomorphic analysis of river catchments and focused field work. We analyzed fluvial and landscape morphometric characteristics in combination with structural, seismicity, and climatic data to better evaluate the tectono-geomorphic history of this transfer zone. Our data reveal significant north-south variations in the degree of extension from the Sawula Basin in the north (mature) to the Chew Bahir Basin in the south (juvenile). First, normalized channel-steepness indices and the spatial arrangement of knickpoints in footwall-draining streams suggest a gradual, southward shift in extensional deformation and recent tectonic activity. Second, based on 1-k(m) radius local relief and mean-hillslope maximum values that are consistent with ksn anomalies, we confirm strain localization within zones of fault interaction. Third, morphometric indices such as hypsometry, basin asymmetry factor, and valley floor width to valley height ratio also indicate a north to south gradient in tectonic activity, highlighting the importance of such a wide transfer zone with diffuse extension linking different rift segments during the break-up of continental crust.}, language = {en} } @article{VossBookhagenSachseetal.2020, author = {Voss, Katalyn A. and Bookhagen, Bodo and Sachse, Dirk and Chadwick, Oliver A.}, title = {Variation of deuterium excess in surface waters across a 5000-m elevation gradient in eastern Nepal}, series = {Journal of hydrology}, volume = {586}, journal = {Journal of hydrology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0022-1694}, doi = {10.1016/j.jhydrol.2020.124802}, pages = {17}, year = {2020}, abstract = {The strong elevation gradient of the Himalaya allows for investigation of altitude and orographic impacts on surface water delta O-18 and delta D stable isotope values. This study differentiates the time- and altitude-variable contributions of source waters to the Arun River in eastern Nepal. It provides isotope data along a 5000-m gradient collected from tributaries as well as groundwater, snow, and glacial-sourced surface waters and time-series data from April to October 2016. We find nonlinear trends in delta O-18 and delta D lapse rates with high-elevation lapse rates (4000-6000 masl) 5-7 times more negative than low-elevation lapse rates (1000-3000 masl). A distinct seasonal signal in delta O-18 and delta D lapse rates indicates time-variable source-water contributions from glacial and snow meltwater as well as precipitation transitions between the Indian Summer Monsoon and Winter Westerly Disturbances. Deuterium excess correlates with the extent of snowpack and tracks melt events during the Indian Summer Monsoon season. Our analysis identifies the influence of snow and glacial melt waters on river composition during low-flow conditions before the monsoon (April/May 2016) followed by a 5-week transition to the Indian Summer Monsoon-sourced rainfall around mid-June 2016. In the post-monsoon season, we find continued influence from glacial melt waters as well as ISM-sourced groundwater.}, language = {en} } @article{CastinoBookhagenDelaTorre2020, author = {Castino, Fabiana and Bookhagen, Bodo and De la Torre, Alejandro}, title = {Atmospheric dynamics of extreme discharge events from 1979 to 2016 in the southern Central Andes}, series = {Climate dynamics : observational, theoretical and computational research on the climate system}, volume = {55}, journal = {Climate dynamics : observational, theoretical and computational research on the climate system}, number = {11-12}, publisher = {Springer}, address = {Berlin ; Heidelberg [u.a.]}, issn = {0930-7575}, doi = {10.1007/s00382-020-05458-1}, pages = {3485 -- 3505}, year = {2020}, abstract = {During the South-American Monsoon season, deep convective systems occur at the eastern flank of the Central Andes leading to heavy rainfall and flooding. We investigate the large- and meso-scale atmospheric dynamics associated with extreme discharge events (> 99.9th percentile) observed in two major river catchments meridionally stretching from humid to semi-arid conditions in the southern Central Andes. Based on daily gauge time series and ERA-Interim reanalysis, we made the following three key observations: (1) for the period 1940-2016 daily discharge exhibits more pronounced variability in the southern, semi-arid than in the northern, humid catchments. This is due to a smaller ratio of discharge magnitudes between intermediate (0.2 year return period) and rare events (20 year return period) in the semi-arid compared to the humid areas; (2) The climatological composites of the 40 largest discharge events showed characteristic atmospheric features of cold surges based on 5-day time-lagged sequences of geopotential height at different levels in the troposphere; (3) A subjective classification revealed that 80\% of the 40 largest discharge events are mainly associated with the north-northeastward migration of frontal systems and 2/3 of these are cold fronts, i.e. cold surges. This work highlights the importance of cold surges and their related atmospheric processes for the generation of heavy rainfall events and floods in the southern Central Andes.}, language = {en} } @misc{PurintonBookhagen2019, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Introducing PebbleCounts}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {783}, issn = {1866-8372}, doi = {10.25932/publishup-43946}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-439468}, pages = {21}, year = {2019}, abstract = {Grain-size distributions are a key geomorphic metric of gravel-bed rivers. Traditional measurement methods include manual counting or photo sieving, but these are achievable only at the 1-10 ㎡ scale. With the advent of drones and increasingly high-resolution cameras, we can now generate orthoimagery over hectares at millimeter to centimeter resolution. These scales, along with the complexity of high-mountain rivers, necessitate different approaches for photo sieving. As opposed to other image segmentation methods that use a watershed approach, our open-source algorithm, PebbleCounts, relies on k-means clustering in the spatial and spectral domain and rapid manual selection of well-delineated grains. This improves grain-size estimates for complex riverbed imagery, without post-processing. We also develop a fully automated method, PebbleCountsAuto, that relies on edge detection and filtering suspect grains, without the k-means clustering or manual selection steps. The algorithms are tested in controlled indoor conditions on three arrays of pebbles and then applied to 12 × 1 ㎡ orthomosaic clips of high-energy mountain rivers collected with a camera-on-mast setup (akin to a low-flying drone). A 20-pixel b-axis length lower truncation is necessary for attaining accurate grain-size distributions. For the k-means PebbleCounts approach, average percentile bias and precision are 0.03 and 0.09 ψ, respectively, for ∼1.16 mm pixel⁻¹ images, and 0.07 and 0.05 ψ for one 0.32 mm pixel⁻¹ image. The automatic approach has higher bias and precision of 0.13 and 0.15 ψ, respectively, for ∼1.16 mm pixel⁻¹ images, but similar values of -0.06 and 0.05 ψ for one 0.32 mm pixel⁻¹ image. For the automatic approach, only at best 70 \% of the grains are correct identifications, and typically around 50 \%. PebbleCounts operates most effectively at the 1 ㎡ patch scale, where it can be applied in ∼5-10 min on many patches to acquire accurate grain-size data over 10-100 ㎡ areas. These data can be used to validate PebbleCountsAuto, which may be applied at the scale of entire survey sites (102-104 ㎡ ). We synthesize results and recommend best practices for image collection, orthomosaic generation, and grain-size measurement using both algorithms.}, language = {en} } @misc{TostEhmelHeidmannetal.2018, author = {Tost, Jordi and Ehmel, Fabian and Heidmann, Frank and Olen, Stephanie M. and Bookhagen, Bodo}, title = {Hazards and accessibility}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {710}, issn = {1866-8372}, doi = {10.25932/publishup-42785}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427853}, pages = {8}, year = {2018}, abstract = {The assessment of natural hazards and risk has traditionally been built upon the estimation of threat maps, which are used to depict potential danger posed by a particular hazard throughout a given area. But when a hazard event strikes, infrastructure is a significant factor that can determine if the situation becomes a disaster. The vulnerability of the population in a region does not only depend on the area's local threat, but also on the geographical accessibility of the area. This makes threat maps by themselves insufficient for supporting real-time decision-making, especially for those tasks that involve the use of the road network, such as management of relief operations, aid distribution, or planning of evacuation routes, among others. To overcome this problem, this paper proposes a multidisciplinary approach divided in two parts. First, data fusion of satellite-based threat data and open infrastructure data from OpenStreetMap, introducing a threat-based routing service. Second, the visualization of this data through cartographic generalization and schematization. This emphasizes critical areas along roads in a simple way and allows users to visually evaluate the impact natural hazards may have on infrastructure. We develop and illustrate this methodology with a case study of landslide threat for an area in Colombia.}, language = {en} } @article{SmithBookhagenRheinwalt2017, author = {Smith, Taylor and Bookhagen, Bodo and Rheinwalt, Aljoscha}, title = {Spatiotemporal patterns of High Mountain Asia's snowmelt season identified with an automated snowmelt detection algorithm, 1987-2016}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {11}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, issn = {1994-0416}, doi = {10.5194/tc-11-2329-2017}, pages = {2329 -- 2343}, year = {2017}, abstract = {High Mountain Asia (HMA) - encompassing the Tibetan Plateau and surrounding mountain ranges - is the primary water source for much of Asia, serving more than a billion downstream users. Many catchments receive the majority of their yearly water budget in the form of snow, which is poorly monitored by sparse in situ weather networks. Both the timing and volume of snowmelt play critical roles in downstream water provision, as many applications - such as agriculture, drinking-water generation, and hydropower - rely on consistent and predictable snowmelt runoff. Here, we examine passive microwave data across HMA with five sensors (SSMI, SSMIS, AMSR-E, AMSR2, and GPM) from 1987 to 2016 to track the timing of the snowmelt season - defined here as the time between maximum passive microwave signal separation and snow clearance. We validated our method against climate model surface temperatures, optical remote-sensing snow-cover data, and a manual control dataset (n = 2100, 3 variables at 25 locations over 28 years); our algorithm is generally accurate within 3-5 days. Using the algorithm-generated snowmelt dates, we examine the spatiotemporal patterns of the snowmelt season across HMA. The climatically short (29-year) time series, along with complex interannual snowfall variations, makes determining trends in snowmelt dates at a single point difficult. We instead identify trends in snowmelt timing by using hierarchical clustering of the passive microwave data to determine trends in self-similar regions. We make the following four key observations. (1) The end of the snowmelt season is trending almost universally earlier in HMA (negative trends). Changes in the end of the snowmelt season are generally between 2 and 8 days decade 1 over the 29-year study period (5-25 days total). The length of the snowmelt season is thus shrinking in many, though not all, regions of HMA. Some areas exhibit later peak signal separation (positive trends), but with generally smaller magnitudes than trends in snowmelt end. (2) Areas with long snowmelt periods, such as the Tibetan Plateau, show the strongest compression of the snowmelt season (negative trends). These trends are apparent regardless of the time period over which the regression is performed. (3) While trends averaged over 3 decades indicate generally earlier snowmelt seasons, data from the last 14 years (2002-2016) exhibit positive trends in many regions, such as parts of the Pamir and Kunlun Shan. Due to the short nature of the time series, it is not clear whether this change is a reversal of a long-term trend or simply interannual variability. (4) Some regions with stable or growing glaciers - such as the Karakoram and Kunlun Shan - see slightly later snowmelt seasons and longer snowmelt periods. It is likely that changes in the snowmelt regime of HMA account for some of the observed heterogeneity in glacier response to climate change. While the decadal increases in regional temperature have in general led to earlier and shortened melt seasons, changes in HMA's cryosphere have been spatially and temporally heterogeneous.}, language = {en} } @article{PurintonBookhagen2017, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Validation of digital elevation models (DEMs) and comparison of geomorphic metrics on the southern Central Andean Plateau}, series = {Earth surface dynamics}, volume = {5}, journal = {Earth surface dynamics}, number = {2}, publisher = {Copernicus Publ.}, address = {G{\"o}ttingen}, issn = {2196-632X}, doi = {10.5194/esurf-5-211-2017}, pages = {211 -- 237}, year = {2017}, abstract = {In this study, we validate and compare elevation accuracy and geomorphic metrics of satellite-derived digital elevation models (DEMs) on the southern Central Andean Plateau. The plateau has an average elevation of 3.7 km and is characterized by diverse topography and relief, lack of vegetation, and clear skies that create ideal conditions for remote sensing. At 30m resolution, SRTM-C, ASTER GDEM2, stacked ASTER L1A stereopair DEM, ALOS World 3D, and TanDEM-X have been analyzed. The higher-resolution datasets include 12m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X DEMs, and 5m ALOS World 3D. These DEMs are state of the art for optical (ASTER and ALOS) and radar (SRTM-C and TanDEM-X) spaceborne sensors. We assessed vertical accuracy by comparing standard deviations of the DEM elevation versus 307 509 differential GPS measurements across 4000m of elevation. For the 30m DEMs, the ASTER datasets had the highest vertical standard deviation at > 6.5 m, whereas the SRTM-C, ALOS World 3D, and TanDEM-X were all < 3.5 m. Higher-resolution DEMs generally had lower uncertainty, with both the 12m TanDEM-X and 5m ALOSWorld 3D having < 2m vertical standard deviation. Analysis of vertical uncertainty with respect to terrain elevation, slope, and aspect revealed the low uncertainty across these attributes for SRTM-C (30 m), TanDEM-X (12-30 m), and ALOS World 3D (5-30 m). Single-CoSSC TerraSAR-X/TanDEM-X 10m DEMs and the 30m ASTER GDEM2 displayed slight aspect biases, which were removed in their stacked counterparts (TanDEM-X and ASTER Stack). Based on low vertical standard deviations and visual inspection alongside optical satellite data, we selected the 30m SRTM-C, 12-30m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X, and 5m ALOS World 3D for geomorphic metric comparison in a 66 km2 catchment with a distinct river knickpoint. Consistent m=n values were found using chi plot channel profile analysis, regardless of DEM type and spatial resolution. Slope, curvature, and drainage area were calculated and plotting schemes were used to assess basin-wide differences in the hillslope-to-valley transition related to the knickpoint. While slope and hillslope length measurements vary little between datasets, curvature displays higher magnitude measurements with fining resolution. This is especially true for the optical 5m ALOS World 3D DEM, which demonstrated high-frequency noise in 2-8 pixel steps through a Fourier frequency analysis. The improvements in accurate space-radar DEMs (e.g., TanDEM-X) for geomorphometry are promising, but airborne or terrestrial data are still necessary for meter-scale analysis.}, language = {en} } @article{PurintonBookhagen2018, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Measuring decadal vertical land-level changes from SRTM-C (2000) and TanDEM-X (∼ 2015) in the south-central Andes}, series = {Earth Surface Dynamics}, volume = {6}, journal = {Earth Surface Dynamics}, publisher = {Copernicus Publ.}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-6-971-2018}, pages = {971 -- 987}, year = {2018}, abstract = {In the arctic and high mountains it is common to measure vertical changes of ice sheets and glaciers via digital elevation model (DEM) differencing. This requires the signal of change to outweigh the noise associated with the datasets. Excluding large landslides, on the ice-free earth the land-level change is smaller in vertical magnitude and thus requires more accurate DEMs for differencing and identification of change. Previously, this has required meter to submeter data at small spatial scales. Following careful corrections, we are able to measure land-level changes in gravel-bed channels and steep hillslopes in the south-central Andes using the SRTM-C (collected in 2000) and the TanDEM-X (collected from 2010 to 2015) near-global 12-30m DEMs. Long-standing errors in the SRTM-C are corrected using the TanDEM-X as a control surface and applying cosine-fit co-registration to remove ∼ 1∕10 pixel (∼ 3m) shifts, fast Fourier transform (FFT) and filtering to remove SRTM-C short- and long-wavelength stripes, and blocked shifting to remove remaining complex biases. The datasets are then differenced and outlier pixels are identified as a potential signal for the case of gravel-bed channels and hillslopes. We are able to identify signals of incision and aggradation (with magnitudes down to ∼ 3m in the best case) in two  > 100km river reaches, with increased geomorphic activity downstream of knickpoints. Anthropogenic gravel excavation and piling is prominently measured, with magnitudes exceeding ±5m (up to  > 10m for large piles). These values correspond to conservative average rates of 0.2 to > 0.5myr-1 for vertical changes in gravel-bed rivers. For hillslopes, since we require stricter cutoffs for noise, we are only able to identify one major landslide in the study area with a deposit volume of 16±0.15×106m3. Additional signals of change can be garnered from TanDEM-X auxiliary layers; however, these are more difficult to quantify. The methods presented can be extended to any region of the world with SRTM-C and TanDEM-X coverage where vertical land-level changes are of interest, with the caveat that remaining vertical uncertainties in primarily the SRTM-C limit detection in steep and complex topography.}, language = {en} } @article{AlonzoBookhagenMcFaddenetal.2015, author = {Alonzo, Michael and Bookhagen, Bodo and McFadden, Joseph P. and Sun, Alex and Roberts, Dar A.}, title = {Mapping urban forest leaf area index with airborne lidar using penetration metrics and allometry}, series = {Remote sensing of environment : an interdisciplinary journal}, volume = {162}, journal = {Remote sensing of environment : an interdisciplinary journal}, publisher = {Elsevier}, address = {New York}, issn = {0034-4257}, doi = {10.1016/j.rse.2015.02.025}, pages = {141 -- 153}, year = {2015}, abstract = {In urban areas, leaf area index (LAI) is a key ecosystem structural attribute with implications for energy and water balance, gas exchange, and anthropogenic energy use. In this study, we estimated LAI spatially using airborne lidar in downtown Santa Barbara, California, USA. We implemented two different modeling approaches. First, we directly estimated effective LAI (LAIe) using scan angle- and clump-corrected lidar laser penetration metrics (LPM). Second, we adapted existing allometric equations to estimate crown structural metrics including tree height and crown base height using lidar. The latter approach allowed for LAI estimates at the individual tree-crown scale. The LPM method, at both high and decimated point densities, resulted in good linear agreement with estimates from ground-based hemispherical photography (r(2) = 0.82, y = 0.99x) using a model that assumed a spherical leaf angle distribution. Within individual tree crown segments, the lidar estimates of crown structure closely paralleled field measurements (e.g., r(2) = 0.87 for crown length). LAI estimates based on the lidar crown measurements corresponded well with estimates from field measurements (r(2) = 0.84, y = 0.97x + 0.10). Consistency of the LPM and allometric lidar methods was also strong at 71 validation plots (r(2) = 0.88) and at 450 additional sample locations across the entire study area (r(2) = 0.72). This level of correspondence exceeded that of the canopy hemispherical photography and allometric, ground-based estimates (r(2) = 0.53). The first-order alignment of these two disparate methods may indicate that the error bounds for mapping LAI in cities are small enough to pursue large scale, spatially explicit estimation. (C) 2015 Elsevier Inc All rights reserved.}, language = {en} } @article{OlenBookhagen2018, author = {Olen, Stephanie M. and Bookhagen, Bodo}, title = {Mapping Damage-Affected Areas after Natural Hazard Events Using Sentinel-1 Coherence Time Series}, series = {remote sensing}, volume = {10}, journal = {remote sensing}, number = {8}, publisher = {Molecular Diversity Preservation International (MDPI)}, address = {Basel}, issn = {2072-4292}, doi = {10.3390/rs10081272}, pages = {1 -- 19}, year = {2018}, abstract = {The emergence of the Sentinel-1A and 1B satellites now offers freely available and widely accessible Synthetic Aperture Radar (SAR) data. Near-global coverage and rapid repeat time (6-12 days) gives Sentinel-1 data the potential to be widely used for monitoring the Earth's surface. Subtle land-cover and land surface changes can affect the phase and amplitude of the C-band SAR signal, and thus the coherence between two images collected before and after such changes. Analysis of SAR coherence therefore serves as a rapidly deployable and powerful tool to track both seasonal changes and rapid surface disturbances following natural disasters. An advantage of using Sentinel-1 C-band radar data is the ability to easily construct time series of coherence for a region of interest at low cost. In this paper, we propose a new method for Potentially Affected Area (PAA) detection following a natural hazard event. Based on the coherence time series, the proposed method (1) determines the natural variability of coherence within each pixel in the region of interest, accounting for factors such as seasonality and the inherent noise of variable surfaces; and (2) compares pixel-by-pixel syn-event coherence to temporal coherence distributions to determine where statistically significant coherence loss has occurred. The user can determine to what degree the syn-event coherence value (e.g., 1st, 5th percentile of pre-event distribution) constitutes a PAA, and integrate pertinent regional data, such as population density, to rank and prioritise PAAs. We apply the method to two case studies, Sarpol-e, Iran following the 2017 Iran-Iraq earthquake, and a landslide-prone region of NW Argentina, to demonstrate how rapid identification and interpretation of potentially affected areas can be performed shortly following a natural hazard event.}, language = {en} } @misc{PurintonBookhagen2018, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Measuring decadal vertical land-level changes from SRTM-C (2000) and TanDEM-X (∼ 2015) in the south-central Andes}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {480}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-420487}, pages = {16}, year = {2018}, abstract = {In the arctic and high mountains it is common to measure vertical changes of ice sheets and glaciers via digital elevation model (DEM) differencing. This requires the signal of change to outweigh the noise associated with the datasets. Excluding large landslides, on the ice-free earth the land-level change is smaller in vertical magnitude and thus requires more accurate DEMs for differencing and identification of change. Previously, this has required meter to submeter data at small spatial scales. Following careful corrections, we are able to measure land-level changes in gravel-bed channels and steep hillslopes in the south-central Andes using the SRTM-C (collected in 2000) and the TanDEM-X (collected from 2010 to 2015) near-global 12-30m DEMs. Long-standing errors in the SRTM-C are corrected using the TanDEM-X as a control surface and applying cosine-fit co-registration to remove ∼ 1∕10 pixel (∼ 3m) shifts, fast Fourier transform (FFT) and filtering to remove SRTM-C short- and long-wavelength stripes, and blocked shifting to remove remaining complex biases. The datasets are then differenced and outlier pixels are identified as a potential signal for the case of gravel-bed channels and hillslopes. We are able to identify signals of incision and aggradation (with magnitudes down to ∼ 3m in the best case) in two  > 100km river reaches, with increased geomorphic activity downstream of knickpoints. Anthropogenic gravel excavation and piling is prominently measured, with magnitudes exceeding ±5m (up to  > 10m for large piles). These values correspond to conservative average rates of 0.2 to > 0.5myr-1 for vertical changes in gravel-bed rivers. For hillslopes, since we require stricter cutoffs for noise, we are only able to identify one major landslide in the study area with a deposit volume of 16±0.15×106m3. Additional signals of change can be garnered from TanDEM-X auxiliary layers; however, these are more difficult to quantify. The methods presented can be extended to any region of the world with SRTM-C and TanDEM-X coverage where vertical land-level changes are of interest, with the caveat that remaining vertical uncertainties in primarily the SRTM-C limit detection in steep and complex topography.}, language = {en} } @misc{SmithBookhagenRheinwalt2017, author = {Smith, Taylor and Bookhagen, Bodo and Rheinwalt, Aljoscha}, title = {Spatiotemporal patterns of High Mountain Asia's snowmelt season identified with an automated snowmelt detection algorithm, 1987-2016}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-403911}, pages = {15}, year = {2017}, abstract = {High Mountain Asia (HMA) - encompassing the Tibetan Plateau and surrounding mountain ranges - is the primary water source for much of Asia, serving more than a billion downstream users. Many catchments receive the majority of their yearly water budget in the form of snow, which is poorly monitored by sparse in situ weather networks. Both the timing and volume of snowmelt play critical roles in downstream water provision, as many applications - such as agriculture, drinking-water generation, and hydropower - rely on consistent and predictable snowmelt runoff. Here, we examine passive microwave data across HMA with five sensors (SSMI, SSMIS, AMSR-E, AMSR2, and GPM) from 1987 to 2016 to track the timing of the snowmelt season - defined here as the time between maximum passive microwave signal separation and snow clearance. We validated our method against climate model surface temperatures, optical remote-sensing snow-cover data, and a manual control dataset (n = 2100, 3 variables at 25 locations over 28 years); our algorithm is generally accurate within 3-5 days. Using the algorithm-generated snowmelt dates, we examine the spatiotemporal patterns of the snowmelt season across HMA. The climatically short (29-year) time series, along with complex interannual snowfall variations, makes determining trends in snowmelt dates at a single point difficult. We instead identify trends in snowmelt timing by using hierarchical clustering of the passive microwave data to determine trends in self-similar regions. We make the following four key observations. (1) The end of the snowmelt season is trending almost universally earlier in HMA (negative trends). Changes in the end of the snowmelt season are generally between 2 and 8 days decade 1 over the 29-year study period (5-25 days total). The length of the snowmelt season is thus shrinking in many, though not all, regions of HMA. Some areas exhibit later peak signal separation (positive trends), but with generally smaller magnitudes than trends in snowmelt end. (2) Areas with long snowmelt periods, such as the Tibetan Plateau, show the strongest compression of the snowmelt season (negative trends). These trends are apparent regardless of the time period over which the regression is performed. (3) While trends averaged over 3 decades indicate generally earlier snowmelt seasons, data from the last 14 years (2002-2016) exhibit positive trends in many regions, such as parts of the Pamir and Kunlun Shan. Due to the short nature of the time series, it is not clear whether this change is a reversal of a long-term trend or simply interannual variability. (4) Some regions with stable or growing glaciers - such as the Karakoram and Kunlun Shan - see slightly later snowmelt seasons and longer snowmelt periods. It is likely that changes in the snowmelt regime of HMA account for some of the observed heterogeneity in glacier response to climate change. While the decadal increases in regional temperature have in general led to earlier and shortened melt seasons, changes in HMA's cryosphere have been spatially and temporally heterogeneous.}, language = {en} } @article{PurintonBookhagen2017, author = {Purinton, Benjamin and Bookhagen, Bodo}, title = {Validation of digital elevation models (DEMs) and comparison of geomorphic metrics on the southern Central Andean Plateau}, series = {Earth surface dynamics}, volume = {5}, journal = {Earth surface dynamics}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {2196-6311}, doi = {10.5194/esurf-5-211-2017}, pages = {211 -- 237}, year = {2017}, abstract = {In this study, we validate and compare elevation accuracy and geomorphic metrics of satellite-derived digital elevation models (DEMs) on the southern Central Andean Plateau. The plateau has an average elevation of 3.7 km and is characterized by diverse topography and relief, lack of vegetation, and clear skies that create ideal conditions for remote sensing. At 30m resolution, SRTM-C, ASTER GDEM2, stacked ASTER L1A stereopair DEM, ALOS World 3D, and TanDEM-X have been analyzed. The higher-resolution datasets include 12m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X DEMs, and 5m ALOS World 3D. These DEMs are state of the art for optical (ASTER and ALOS) and radar (SRTM-C and TanDEM-X) spaceborne sensors. We assessed vertical accuracy by comparing standard deviations of the DEM elevation versus 307 509 differential GPS measurements across 4000m of elevation. For the 30m DEMs, the ASTER datasets had the highest vertical standard deviation at > 6.5 m, whereas the SRTM-C, ALOS World 3D, and TanDEM-X were all < 3.5 m. Higher-resolution DEMs generally had lower uncertainty, with both the 12m TanDEM-X and 5m ALOSWorld 3D having < 2m vertical standard deviation. Analysis of vertical uncertainty with respect to terrain elevation, slope, and aspect revealed the low uncertainty across these attributes for SRTM-C (30 m), TanDEM-X (12-30 m), and ALOS World 3D (5-30 m). Single-CoSSC TerraSAR-X/TanDEM-X 10m DEMs and the 30m ASTER GDEM2 displayed slight aspect biases, which were removed in their stacked counterparts (TanDEM-X and ASTER Stack). Based on low vertical standard deviations and visual inspection alongside optical satellite data, we selected the 30m SRTM-C, 12-30m TanDEM-X, 10m single-CoSSC TerraSAR-X/TanDEM-X, and 5m ALOS World 3D for geomorphic metric comparison in a 66 km2 catchment with a distinct river knickpoint. Consistent m = n values were found using chi plot channel profile analysis, regardless of DEM type and spatial resolution. Slope, curvature, and drainage area were calculated and plotting schemes were used to assess basin-wide differences in the hillslope-to-valley transition related to the knickpoint. While slope and hillslope length measurements vary little between datasets, curvature displays higher magnitude measurements with fining resolution. This is especially true for the optical 5m ALOS World 3D DEM, which demonstrated high-frequency noise in 2-8 pixel steps through a Fourier frequency analysis. The improvements in accurate space-radar DEMs (e. g., TanDEM-X) for geomorphometry are promising, but airborne or terrestrial data are still necessary for meter-scale analysis.}, language = {en} } @article{MilewskiChabrillatBookhagen2020, author = {Milewski, Robert and Chabrillat, Sabine and Bookhagen, Bodo}, title = {Analyses of Namibian Seasonal Salt Pan Crust Dynamics and Climatic Drivers Using Landsat 8 Time-Series and Ground Data}, series = {Remote Sensing}, journal = {Remote Sensing}, number = {3}, publisher = {MDPI}, address = {Basel}, issn = {2072-4292}, doi = {10.3390/rs12030474}, pages = {24}, year = {2020}, abstract = {Salt pans are highly dynamic environments that are difficult to study by in situ methods because of their harsh climatic conditions and large spatial areas. Remote sensing can help to elucidate their environmental dynamics and provide important constraints regarding their sedimentological, mineralogical, and hydrological evolution. This study utilizes spaceborne multitemporal multispectral optical data combined with spectral endmembers to document spatial distribution of surface crust types over time on the Omongwa pan located in the Namibian Kalahari. For this purpose, 49 surface samples were collected for spectral and mineralogical characterization during three field campaigns (2014-2016) reflecting different seasons and surface conditions of the salt pan. An approach was developed to allow the spatiotemporal analysis of the salt pan crust dynamics in a dense time-series consisting of 77 Landsat 8 cloud-free scenes between 2014 and 2017, covering at least three major wet-dry cycles. The established spectral analysis technique Sequential Maximum Angle Convex Cone (SMACC) extraction method was used to derive image endmembers from the Landsat time-series stack. Evaluation of the extracted endmember set revealed that the multispectral data allowed the differentiation of four endmembers associated with mineralogical mixtures of the crust's composition in dry conditions and three endmembers associated with flooded or muddy pan conditions. The dry crust endmember spectra have been identified in relation to visible, near infrared, and short-wave infrared (VNIR-SWIR) spectroscopy and X-ray diffraction (XRD) analyses of the collected surface samples. According these results, the spectral endmembers are interpreted as efflorescent halite crust, mixed halite-gypsum crust, mixed calcite quartz sepiolite crust, and gypsum crust. For each Landsat scene the spatial distribution of these crust types was mapped with the Spectral Angle Mapper (SAM) method and significant spatiotemporal dynamics of the major surface crust types were observed. Further, the surface crust dynamics were analyzed in comparison with the pan's moisture regime and other climatic parameters. The results show that the crust dynamics are mainly driven by flooding events in the wet season, but are also influenced by temperature and aeolian activity in the dry season. The approach utilized in this study combines the advantages of multitemporal satellite data for temporal event characterization with advantages from hyperspectral methods for the image and ground data analyses that allow improved mineralogical differentiation and characterization.}, language = {en} } @article{BrellSeglGuanteretal.2017, author = {Brell, Maximilian and Segl, Karl and Guanter, Luis and Bookhagen, Bodo}, title = {Hyperspectral and Lidar Intensity Data Fusion: A Framework for the Rigorous Correction of Illumination, Anisotropic Effects, and Cross Calibration}, series = {IEEE transactions on geoscience and remote sensing}, volume = {55}, journal = {IEEE transactions on geoscience and remote sensing}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {0196-2892}, doi = {10.1109/TGRS.2017.2654516}, pages = {2799 -- 2810}, year = {2017}, abstract = {The fusion of hyperspectral imaging (HSI) sensor and airborne lidar scanner (ALS) data provides promising potential for applications in environmental sciences. Standard fusion approaches use reflectance information from the HSI and distance measurements from the ALS to increase data dimen-sionality and geometric accuracy. However, the potential for data fusion based on the respective intensity information of the complementary active and passive sensor systems is high and not yet fully exploited. Here, an approach for the rigorous illumination correction of HSI data, based on the radiometric cross-calibrated return intensity information of ALS data, is presented. The cross calibration utilizes a ray tracing-based fusion of both sensor measurements by intersecting their particular beam shapes. The developed method is capable of compensating for the drawbacks of passive HSI systems, such as cast and cloud shadowing effects, illumination changes over time, across track illumination, and partly anisotropy effects. During processing, spatial and temporal differences in illumination patterns are detected and corrected over the entire HSI wavelength domain. The improvement in the classification accuracy of urban and vegetation surfaces demonstrates the benefit and potential of the proposed HSI illumination correction. The presented approach is the first step toward the rigorous in-flight fusion of passive and active system characteristics, enabling new capabilities for a variety of applications.}, language = {en} } @article{MonteroLopezStreckerSchildgenetal.2014, author = {Montero-Lopez, Carolina and Strecker, Manfred and Schildgen, Taylor F. and Hongn, Fernando D. and Guzman, Silvina and Bookhagen, Bodo and Sudo, Masafumi}, title = {Local high relief at the southern margin of the Andean plateau by 9 Ma: evidence from ignimbritic valley fills and river incision}, series = {Terra nova}, volume = {26}, journal = {Terra nova}, number = {6}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0954-4879}, doi = {10.1111/ter.12120}, pages = {454 -- 460}, year = {2014}, abstract = {A valley-filling ignimbrite re-exposed through subsequent river incision at the southern margin of the Andean (Puna) plateau preserves pristine geological evidence of pre-late Miocene palaeotopography in the north western Argentine Andes. Our new Ar-40/(39) Ar dating of the Las Papas Ignimbrites yields a plateau age of 9.24 +/- 0.03 Ma, indicating valley-relief and orographic-barrier conditions comparable to the present-day. A later infill of Plio-Pleistocene coarse conglomerates has been linked to wetter conditions, but resulted in no additional net incision of the Las Papas valley, considering that the base of the ignimbrite remains unexposed in the valley bottom. Our observations indicate that at least 550 m of local plateau margin relief (and likely > 2 km) existed by 9 Ma at the southern Puna margin, which likely aided the efficiency of the orographic barrier to rainfall along the eastern and south eastern flanks of the Puna and causes aridity in the plateau interior.}, language = {en} } @article{ThiedeEhlersBookhagenetal.2009, author = {Thiede, Rasmus Christoph and Ehlers, Todd and Bookhagen, Bodo and Strecker, Manfred}, title = {Erosional variability along the northwest Himalaya}, issn = {0148-0227}, doi = {10.1029/2008jf001010}, year = {2009}, abstract = {Erosional exhumation and topography in mountain belts are temporally and spatially variable over million year timescales because of changes in both the location of deformation and climate. We investigate spatiotemporal variations in exhumation across a 150 x 250 km compartment of the NW Himalaya, India. Twenty-four new and 241 previously published apatite and zircon fission track and white mica Ar-40/Ar-39 ages are integrated with a 1-D numerical model to quantify rates and timing of exhumation alongstrike of several major structures in the Lesser, High, and Tethyan Himalaya. Analysis of thermochronometer data suggests major temporal variations in exhumation occurred in the early middle Miocene and at the Plio-Pleistocene transition. (1) Most notably, exhumation rates for the northern High Himalayan compartments were high (2-3 mm a(-1)) between similar to 23-19 and similar to 3-0 Ma and low (0.5-0.7 mm a(-1)) in between similar to 19-3 Ma. (2) Along the southern High Himalayan slopes, however, high exhumation rates of 1-2 mm a(-1) existed since 11 Ma. (3) Our thermochronology data sets are poorly correlated with present-day rainfall, local relief, and specific stream power which may likely result from (1) a lack of sensitivity of changes in crustal cooling to spatial variations in erosion at high exhumation rates (>similar to 1 mm a(-1)), (2) spatiotemporal variation in erosion not mimicking the present-day topographic or climatic conditions, or (3) the thermochronometer samples in this region having cooled under topography that only weakly resembled the modern-day topography.}, language = {en} } @article{BookhagenEchtlerMelnicketal.2006, author = {Bookhagen, Bodo and Echtler, Helmut Peter and Melnick, Daniel and Strecker, Manfred and Spencer, Joel Q. G.}, title = {Using uplifted Holocene beach berms for paleoseismic analysis on the Santa Maria Island, south-central Chile}, issn = {0094-8276}, doi = {10.1029/2006gl026734}, year = {2006}, abstract = {Major earthquakes ( M > 8) have repeatedly ruptured the Nazca-South America plate interface of south-central Chile involving meter scale land-level changes. Earthquake recurrence intervals, however, extending beyond limited historical records are virtually unknown, but would provide crucial data on the tectonic behavior of forearcs. We analyzed the spatiotemporal pattern of Holocene earthquakes on Santa Maria Island (SMI; 37 degrees S), located 20 km off the Chilean coast and approximately 70 km east of the trench. SMI hosts a minimum of 21 uplifted beach berms, of which a subset were dated to calculate a mean uplift rate of 2.3 +/- 0.2 m/ky and a tilting rate of 0.022 +/- 0.002 degrees/ky. The inferred recurrence interval of strandline-forming earthquakes is similar to 180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is similar to 0.4 m per event}, language = {en} } @article{BookhagenThiedeStrecker2005, author = {Bookhagen, Bodo and Thiede, Rasmus Christoph and Strecker, Manfred}, title = {Abnormal monsoon years and their control on erosion and sediment flux in the high, and northwest Himalaya}, year = {2005}, abstract = {The interplay between topography and Indian summer monsoon circulation profoundly controls precipitation distribution, sediment transport, and river discharge along the Southern Himalayan Mountain Front (SHF). The Higher Himalayas form a major orographic barrier that separates humid sectors to the south and and regions to the north. During the Indian summer monsoon, vortices transport moisture from the Bay of Bengal, swirl along the SHF to the northwest, and cause heavy rainfall when colliding with the mountain front. In the eastern and central parts of the Himalaya, precipitation measurements derived from passive microwave analysis (SSM/I) show a strong gradient, with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen. The end of the monsoonal conveyer belt is near the Sutlej Valley in the NW Himalaya, where precipitation is lower and rainfall maxima move to lower elevations. This region thus comprises a climatic transition zone that is very sensitive to changes in Indian summer monsoon strength. To constrain magnitude, temporal, and spatial distribution of precipitation, we analyzed high-resolution passive microwave data from the last decade and identified an abnormal monsoon year (AMY) in 2002. During the 2002 AMY, violent rainstorms conquered orographic barriers and penetrated far into otherwise and regions in the northwest Himalaya at elevations in excess of 3 km asl. While precipitation in these regions was significantly increased and triggered extensive erosional processes (i.e., debris flows) on sparsely vegetated, steep hillslopes, mean rainfall along the low to medium elevations was not significantly greater in magnitude. This shift may thus play an important role in the overall sediment flux toward the Himalayan foreland. Using extended precipitation and sediment flux records for the last century, we show that these events have a decadal recurrence interval during the present-day monsoon circulation. Hence, episodically occurring AMYs control geomorphic processes primarily in the high-elevation and sectors of the orogen, while annual recurring monsoonal rainfall distribution dominates erosion in the low- to medium- elevation parts along the SHF. (C) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{BookhagenThiedeStrecker2005, author = {Bookhagen, Bodo and Thiede, Rasmus Christoph and Strecker, Manfred}, title = {Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya}, issn = {0091-7613}, year = {2005}, abstract = {The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sediment records. However, the mechanisms of long-lived (2-5 k.y.) intensified monsoon phases, the related changes in precipitation distribution, and their effect on landscape evolution and sedimentation rates are not yet well understood. The and high-elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal (i.e., strengthened) monsoon seasons. Analogous to present-day rainfall anomalies, enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure. We associate landslide triggering during intensified monsoon phases with enhanced precipitation, discharge, and sediment flux leading to an increase in pore-water pressure, lateral scouring of rivers, and over- steepening of hillslopes, eventually resulting in failure of slopes and exceptionally large mass movements. Here we use lacustrine deposits related to spatially and temporally clustered large landslides (>0.5 km(3)) in the Sutlej Valley region of the northwest Himalaya to calculate sedimentation rates and to infer rainfall patterns during late Pleistocene (29-24 ka) and Holocene (10-4 ka) intensified monsoon phases. Compared to present-day sediment-flux measurements, a fivefold increase in sediment-transport rates recorded by sediments in landslide-dammed lakes characterized these episodes of high climatic variability. These changes thus emphasize the pronounced imprint of millennial-scale climate change on surface processes and landscape evolution}, language = {en} } @article{BookhagenFleitmannNishiizumietal.2006, author = {Bookhagen, Bodo and Fleitmann, Dominik and Nishiizumi, Kunihiko and Strecker, Manfred and Thiede, Rasmus Christoph}, title = {Holocene monsoonal dynamics and fluvial terrace formation in the northwest Himalaya, India}, issn = {0091-7613}, doi = {10.1130/G22698.1}, year = {2006}, abstract = {Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill}, language = {en} } @article{ScherlerBookhagenStrecker2011, author = {Scherler, Dirk and Bookhagen, Bodo and Strecker, Manfred}, title = {Hillslope-glacier coupling the interplay of topography and glacial dynamics in High Asia}, series = {Journal of geophysical research : Earth surface}, volume = {116}, journal = {Journal of geophysical research : Earth surface}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0148-0227}, doi = {10.1029/2010JF001751}, pages = {21}, year = {2011}, abstract = {High Asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas. Associated differences in glacial cover and dynamics allow a first-order determination of the dominant processes shaping these landscapes. Here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across High Asia using digital elevation analysis and remotely sensed glacier surface velocities. Glaciers situated in low-relief areas on the Tibetan Plateau are mainly nourished by direct snowfall, have little or no debris cover, and have a relatively symmetrical distribution of velocities along their length. In contrast, avalanche-fed glaciers with steep accumulation areas, which occur at the deeply incised edges of the Tibetan Plateau, are heavily covered with supraglacial debris, and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris-mantled lower parts. The downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase. Our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover. The melt-lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line. However, slow velocities limit the erosion potential of such glaciers, and their main landscape-shaping contribution may simply be the evacuation of debris from the base of glacial headwalls, which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes. We propose a conceptual model in which glacially influenced plateau margins evolve from low-relief to high-relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay.}, language = {en} } @article{HainStreckerBookhagenetal.2011, author = {Hain, Mathis P. and Strecker, Manfred and Bookhagen, Bodo and Alonso, Ricardo N. and Pingel, H. and Schmitt, Axel K.}, title = {Neogene to quaternary broken foreland formation and sedimentation dynamics in the Andes of NW Argentina (25 degrees S)}, series = {Tectonics}, volume = {30}, journal = {Tectonics}, number = {11}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0278-7407}, doi = {10.1029/2010TC002703}, pages = {27}, year = {2011}, abstract = {The northwest Argentine Andes constitute a premier natural laboratory to assess the complex interactions between isolated uplifts, orographic precipitation gradients, and related erosion and sedimentation patterns. Here we present new stratigraphic observations and age information from intermontane basin sediments to elucidate the Neogene to Quaternary shortening history and associated sediment dynamics of the broken Salta foreland. This part of the Andean orogen, which comprises an array of basement-cored range uplifts, is located at similar to 25 degrees S and lies to the east of the arid intraorogenic Altiplano/Puna plateau. In the Salta foreland, spatially and temporally disparate range uplift along steeply dipping inherited faults has resulted in foreland compartmentalization with steep basin-tobasin precipitation gradients. Sediment architecture and facies associations record a three-phase (similar to 10, similar to 5, and <2 Ma), east directed, yet unsystematic evolution of shortening, foreland fragmentation, and ensuing changes in precipitation and sediment transport. The provenance signatures of these deposits reflect the trapping of sediments in the intermontane basins of the Andean hinterland, as well as the evolution of a severed fluvial network. Present-day moisture supply to the hinterland is determined by range relief and basin elevation. The conspiring effects of range uplift and low rainfall help the entrapment and long-term storage of sediments, ultimately raising basin elevation in the hinterland, which may amplify aridification in the orogen interior.}, language = {en} } @article{ScherlerBookhagenStrecker2011, author = {Scherler, Dirk and Bookhagen, Bodo and Strecker, Manfred}, title = {Spatially variable response of Himalayan glaciers to climate change affected by debris cover}, series = {Nature geoscience}, volume = {4}, journal = {Nature geoscience}, number = {3}, publisher = {Nature Publ. Group}, address = {New York}, issn = {1752-0894}, doi = {10.1038/NGEO1068}, pages = {156 -- 159}, year = {2011}, abstract = {Controversy about the current state and future evolution of Himalayan glaciers has been stirred up by erroneous statements in the fourth report by the Intergovernmental Panel on Climate Change(1,2). Variable retreat rates(3-6) and a paucity of glacial mass-balance data(7,8) make it difficult to develop a coherent picture of regional climate-change impacts in the region. Here, we report remotely-sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 that provide evidence for strong spatial variations in glacier behaviour which are linked to topography and climate. More than 65\% of the monsoon-influenced glaciers that we observed are retreating, but heavily debris-covered glaciers with stagnant low-gradient terminus regions typically have stable fronts. Debris-covered glaciers are common in the rugged central Himalaya, but they are almost absent in subdued landscapes on the Tibetan Plateau, where retreat rates are higher. In contrast, more than 50\% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability(9,10) or global sea level(11).}, language = {en} } @article{DeyThiedeSchildgenetal.2016, author = {Dey, Saptarshi and Thiede, Rasmus Christoph and Schildgen, Taylor F. and Wittmann, Hella and Bookhagen, Bodo and Scherler, Dirk and Jain, Vikrant and Strecker, Manfred}, title = {Climate-driven sediment aggradation and incision since the late Pleistocene in the NW Himalaya, India}, series = {Earth \& planetary science letters}, volume = {449}, journal = {Earth \& planetary science letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-821X}, doi = {10.1016/j.epsl.2016.05.050}, pages = {321 -- 331}, year = {2016}, abstract = {Deciphering the response of sediment routing systems to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Kangra Basin (northwest Sub-Himalaya, India), upper Pleistocene to Holocene alluvial fills and fluvial terraces record periodic fluctuations of sediment supply and transport capacity on timescales of 10(3) to 10(5) yr. To evaluate the potential influence of climate change on these fluctuations, we compare the timing of aggradation and incision phases recorded within remnant alluvial fans and terraces with climate archives. New surface-exposure dating of six terrace levels with in-situ cosmogenic Be-10 indicates the onset of incision phases. Two terrace surfaces from the highest level (T1) sculpted into the oldest preserved alluvial fan (AF1) date back to 53.4 +/- 3.2 ka and 43.0 +/- 2.7 ka (1 sigma). T2 surfaces sculpted into the remnants of AF1 have exposure ages of 18.6 +/- 1.2 ka and 15.3 +/- 0.9 ka, while terraces sculpted into the upper Pleistocene-Holocene fan (AF2) provide ages of 9.3 +/- 0.4 ka (T3), 7.1 +/- 0.4 ka (T4), 5.2 +/- 0.4 ka (T5) and 3.6 +/- 0.2 ka (T6). Together with previously published OSL ages yielding the timing of aggradation, we find a correlation between variations in sediment transport with oxygen-isotope records from regions affected by the Indian Summer Monsoon. During periods of increased monsoon intensity and post-Last Glacial Maximum glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux, whereas periods of weakened monsoon intensity or lower sediment supply coincide with incision. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} } @article{DeyThiedeSchildgenetal.2016, author = {Dey, Saptarshi and Thiede, Rasmus Christoph and Schildgen, Taylor F. and Wittmann, Hella and Bookhagen, Bodo and Scherler, Dirk and Strecker, Manfred}, title = {Holocene internal shortening within the northwest Sub-Himalaya: Out-of-sequence faulting of the Jwalamukhi Thrust, India}, series = {Tectonics}, volume = {35}, journal = {Tectonics}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0278-7407}, doi = {10.1002/2015TC004002}, pages = {2677 -- 2697}, year = {2016}, abstract = {The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust, is thought to accommodate most of the ongoing crustal shortening in the Sub-Himalaya. Steepened longitudinal river profile segments, terrace offsets, and back-tilted fluvial terraces within the Kangra reentrant of the NW Sub-Himalaya suggest Holocene activity of the Jwalamukhi Thrust (JMT) and other thrust faults that may be associated with strain partitioning along the toe of the Himalayan wedge. To assess the shortening accommodated by the JMT, we combine morphometric terrain analyses with in situ Be-10-based surface-exposure dating of the deformed terraces. Incision into upper Pleistocene sediments within the Kangra Basin created two late Pleistocene terrace levels (T1 and T2). Subsequent early Holocene aggradation shortly before similar to 10ka was followed by episodic reincision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.10.9ka. A vertical offset of 445m of terrace T3 across the JMT indicates a shortening rate of 5.60.8 to 7.51.1mma(-1) over the last similar to 10ka. This result suggests that thrusting along the JMT accommodates 40-60\% of the total Sub-Himalayan shortening in the Kangra reentrant over the Holocene. We speculate that this out-of-sequence shortening may have been triggered or at least enhanced by late Pleistocene and Holocene erosion of sediments from the Kangra Basin.}, language = {en} } @article{CastinoBookhagenStrecker2016, author = {Castino, Fabiana and Bookhagen, Bodo and Strecker, Manfred}, title = {River-discharge dynamics in the Southern Central Andes and the 1976-77 global climate shift}, series = {Geophysical research letters}, volume = {43}, journal = {Geophysical research letters}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1002/2016GL070868}, pages = {11679 -- 11687}, year = {2016}, abstract = {Recent studies have shown that the 1976-77 global climate shift strongly affected the South American climate. In our study, we observed a link between this climate shift and river-discharge variability in the subtropical Southern Central Andes. We analyzed the daily river-discharge time series between 1940 and 1999 from small to medium mountain drainage basins (10(2)-10(4) km(2) ) across a steep climatic and topographic gradient. We document that the discharge frequency distribution changed significantly, with higher percentiles exhibiting more pronounced trends. A change point between 1971 and 1977 marked an intensification of the hydrological cycle, which resulted in increased river discharge. In the upper Rio Bermejo basin of the northernmost Argentine Andes, the mean annual discharge increased by 40\% over 7 years. Our findings are important for flood risk management in areas impacted by the 1976-77 climate shift; discharge frequency distribution analysis provides important insights into the variability of the hydrological cycle in the Andean realm.}, language = {en} } @article{TeshebaevaEchtlerBookhagenetal.2019, author = {Teshebaeva, Kanayim and Echtler, Helmut and Bookhagen, Bodo and Strecker, Manfred}, title = {Deep-seated gravitational slope deformation (DSGSD) and slow-moving landslides in the southern Tien Shan Mountains: new insights from InSAR, tectonic and geomorphic analysis}, series = {Earth surface processes and landforms : the journal of the British Geomorphological Research Group}, volume = {44}, journal = {Earth surface processes and landforms : the journal of the British Geomorphological Research Group}, number = {12}, publisher = {Wiley}, address = {Hoboken}, issn = {0197-9337}, doi = {10.1002/esp.4648}, pages = {2333 -- 2348}, year = {2019}, abstract = {We investigated deep-seated gravitational slope deformation (DSGSD) and slow mass movements in the southern Tien Shan Mountains front using synthetic aperture radar (SAR) time-series data obtained by the ALOS/PALSAR satellite. DSGSD evolves with a variety of geomorphological changes (e.g. valley erosion, incision of slope drainage networks) over time that affect earth surfaces and, therefore, often remain unexplored. We analysed 118 interferograms generated from 20 SAR images that covered about 900 km(2). To understand the spatial pattern of the slope movements and to identify triggering parameters, we correlated surface dynamics with the tectono-geomorphic processes and lithologic conditions of the active front of the Alai Range. We observed spatially continuous, constant hillslope movements with a downslope speed of approximately 71 mm year(-1) velocity. Our findings suggest that the lithological and structural framework defined by protracted deformation was the main controlling factor for sustained relief and, consequently, downslope mass movements. The analysed structures revealed integration of a geological/structural setting with the superposition of Cretaceous-Paleogene alternating carbonatic and clastic sedimentary structures as the substratum for younger, less consolidated sediments. This type of structural setting causes the development of large-scale, gravity-driven DSGSD and slow mass movement. Surface deformations with clear scarps and multiple crest lines triggered planes for large-scale deep mass creeps, and these were related directly to active faults and folds in the geologic structures. Our study offers a new combination of InSAR techniques and structural field observations, along with morphometric and seismologic correlations, to identify and quantify slope instability phenomena along a tectonically active mountain front. These results contribute to an improved natural risk assessment in these structures.}, language = {en} } @article{YildirimSchildgenEchtleretal.2013, author = {Yildirim, Cengiz and Schildgen, Taylor F. and Echtler, Helmut Peter and Melnick, Daniel and Bookhagen, Bodo and Ciner, T. Attila and Niedermann, Samuel and Merchel, Silke and Martschini, Martin and Steier, Peter and Strecker, Manfred}, title = {Tectonic implications of fluvial incision and pediment deformation at the northern margin of the Central Anatolian Plateau based on multiple cosmogenic nuclides}, series = {Tectonics}, volume = {32}, journal = {Tectonics}, number = {5}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0278-7407}, doi = {10.1002/tect.20066}, pages = {1107 -- 1120}, year = {2013}, abstract = {We document Quaternary fluvial incision driven by fault-controlled surface deformation in the inverted intermontane G{\"o}kirmak Basin in the Central Pontide mountains along the northern margin of the Central Anatolian Plateau. In-situ-produced Be-10, Ne-21, and Cl-36 concentrations from gravel-covered fluvial terraces and pediment surfaces along the trunk stream of the basin (the G{\"o}kirmak River) yield model exposure ages ranging from 71ka to 34645ka and average fluvial incision rates over the past similar to 350ka of 0.280.01mm a(-1). Similarities between river incision rates and coastal uplift rates at the Black Sea coast suggest that regional uplift is responsible for the river incision. Model exposure ages of deformed pediment surfaces along tributaries of the trunk stream range from 605ka to 110 +/- 10ka, demonstrating that the thrust faults responsible for pediment deformation were active after those times and were likely active earlier as well as explaining the topographic relief of the region. Together, our data demonstrate cumulative incision that is linked to active internal shortening and uplift of similar to 0.3mm a(-1) in the Central Pontide orogenic wedge, which may ultimately contribute to the lateral growth of the northern Anatolian Plateau.}, language = {en} } @article{ScherlerBookhagenWulfetal.2015, author = {Scherler, Dirk and Bookhagen, Bodo and Wulf, Hendrik and Preusser, Frank and Strecker, Manfred}, title = {Increased late Pleistocene erosion rates during fluvial aggradation in the Garhwal Himalaya, northern India}, series = {Earth \& planetary science letters}, volume = {428}, journal = {Earth \& planetary science letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-821X}, doi = {10.1016/j.epsl.2015.06.034}, pages = {255 -- 266}, year = {2015}, abstract = {The response of surface processes to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Himalaya, most large rivers feature prominent fill terraces that record an imbalance between sediment supply and transport capacity, presumably due to past fluctuations in monsoon precipitation and/or effects of glaciation at high elevation. Here, we present volume estimates, chronological constraints, and Be-10-derived paleo-erosion rates from a prominent valley fill in the Yamuna catchment, Garhwal Himalaya, to elucidate the coupled response of rivers and hillslopes to Pleistocene climate change. Although precise age control is complicated due to methodological problems, the new data support formation of the valley fill during the late Pleistocene and its incision during the Holocene. We interpret this timing to indicate that changes in discharge and river-transport capacity were major controls. Compared to the present day, late Pleistocene hillslope erosion rates were higher by a factor of similar to 2-4, but appear to have decreased during valley aggradation. The higher late Pleistocene erosion rates are largely unrelated to glacial erosion and could be explained by enhanced sediment production on steep hillslopes due to increased periglacial activity that declined as temperatures increased. Alternatively, erosion rates that decrease during valley aggradation are also consistent with reduced landsliding from threshold hillslopes as a result of rising base levels. In that case, the similarity of paleo-erosion rates near the end of the aggradation period with modern erosion rates might imply that channels and hillslopes are not yet fully coupled everywhere and that present-day hillslope erosion rates may underrepresent long-term incision rates. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @article{AcostaSchildgenClarkeetal.2015, author = {Acosta, Veronica Torres and Schildgen, Taylor F. and Clarke, Brian A. and Scherler, Dirk and Bookhagen, Bodo and Wittmann, Hella and von Blanckenburg, Friedhelm and Strecker, Manfred}, title = {Effect of vegetation cover on millennial-scale landscape denudation rates in East Africa}, series = {Lithosphere}, volume = {7}, journal = {Lithosphere}, number = {4}, publisher = {American Institute of Physics}, address = {Boulder}, issn = {1941-8264}, doi = {10.1130/L402.1}, pages = {408 -- 420}, year = {2015}, abstract = {The mechanisms by which climate and vegetation affect erosion rates over various time scales lie at the heart of understanding landscape response to climate change. Plot-scale field experiments show that increased vegetation cover slows erosion, implying that faster erosion should occur under low to moderate vegetation cover. However, demonstrating this concept over long time scales and across landscapes has proven to be difficult, especially in settings complicated by tectonic forcing and variable slopes. We investigate this problem by measuring cosmogenic Be-10-derived catchment-mean denudation rates across a range of climate zones and hillslope gradients in the Kenya Rift, and by comparing our results with those published from the Rwenzori Mountains of Uganda. We find that denudation rates from sparsely vegetated parts of the Kenya Rift are up to 0.13 mm/yr, while those from humid and more densely vegetated parts of the Kenya Rift flanks and the Rwenzori Mountains reach a maximum of 0.08 mm/yr, despite higher median hillslope gradients. While differences in lithology and recent land-use changes likely affect the denudation rates and vegetation cover values in some of our studied catchments, hillslope gradient and vegetation cover appear to explain most of the variation in denudation rates across the study area. Our results support the idea that changing vegetation cover can contribute to complex erosional responses to climate or land-use change and that vegetation cover can play an important role in determining the steady-state slopes of mountain belts through its stabilizing effects on the land surface.}, language = {en} } @phdthesis{Bookhagen2004, author = {Bookhagen, Bodo}, title = {Late quaternary climate changes and landscape evolution in the Northwest Himalaya : geomorphologic processes in the Indian Summer Monsoon Domain}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001956}, school = {Universit{\"a}t Potsdam}, year = {2004}, abstract = {The India-Eurasia continental collision zone provides a spectacular example of active mountain building and climatic forcing. In order to quantify the critically important process of mass removal, I analyzed spatial and temporal precipitation patterns of the oscillating monsoon system and their geomorphic imprints. I processed passive microwave satellite data to derive high-resolution rainfall estimates for the last decade and identified an abnormal monsoon year in 2002. During this year, precipitation migrated far into the Sutlej Valley in the northwestern part of the Himalaya and reached regions behind orographic barriers that are normally arid. There, sediment flux, mean basin denudation rates, and channel-forming processes such as erosion by debris-flows increased significantly. Similarly, during the late Pleistocene and early Holocene, solar forcing increased the strength of the Indian summer monsoon for several millennia and presumably lead to analogous precipitation distribution as were observed during 2002. However, the persistent humid conditions in the steep, high-elevation parts of the Sutlej River resulted in deep-seated landsliding. Landslides were exceptionally large, mainly due to two processes that I infer for this time: At the onset of the intensified monsoon at 9.7 ka BP heavy rainfall and high river discharge removed material stored along the river, and lowered the baselevel. Second, enhanced discharge, sediment flux, and increased pore-water pressures along the hillslopes eventually lead to exceptionally large landslides that have not been observed in other periods. The excess sediments that were removed from the upstream parts of the Sutlej Valley were rapidly deposited in the low-gradient sectors of the lower Sutlej River. Timing of downcutting correlates with centennial-long weaker monsoon periods that were characterized by lower rainfall. I explain this relationship by taking sediment flux and rainfall dynamics into account: High sediment flux derived from the upstream parts of the Sutlej River during strong monsoon phases prevents fluvial incision due to oversaturation the fluvial sediment-transport capacity. In contrast, weaker monsoons result in a lower sediment flux that allows incision in the low-elevation parts of the Sutlej River.}, language = {en} } @article{OlenBookhagenHoffmannetal.2015, author = {Olen, Stephanie M. and Bookhagen, Bodo and Hoffmann, Bernd and Sachse, Dirk and Adhikari, Danda P. and Strecker, Manfred}, title = {Understanding erosion rates in the Himalayan orogen: A case study from the Arun Valley}, series = {Journal of geophysical research : Earth surface}, volume = {120}, journal = {Journal of geophysical research : Earth surface}, number = {10}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1002/2014JF003410}, pages = {2080 -- 2102}, year = {2015}, abstract = {Understanding the rates and pattern of erosion is a key aspect of deciphering the impacts of climate and tectonics on landscape evolution. Denudation rates derived from terrestrial cosmogenic nuclides (TCNs) are commonly used to quantify erosion and bridge tectonic (Myr) and climatic (up to several kiloyears) time scales. However, how the processes of erosion in active orogens are ultimately reflected in Be-10 TCN samples remains a topic of discussion. We investigate this problem in the Arun Valley of eastern Nepal with 34 new Be-10-derived catchment-mean denudation rates. The Arun Valley is characterized by steep north-south gradients in topography and climate. Locally, denudation rates increase northward, from <0.2mmyr(-1) to similar to 1.5mmyr(-1) in tributary samples, while main stem samples appear to increase downstream from similar to 0.2mmyr(-1) at the border with Tibet to 0.91mmyr(-1) in the foreland. Denudation rates most strongly correlate with normalized channel steepness (R-2=0.67), which has been commonly interpreted to indicate tectonic activity. Significant downstream decrease of Be-10 concentration in the main stem Arun suggests that upstream sediment grains are fining to the point that they are operationally excluded from the processed sample. This results in Be-10 concentrations and denudation rates that do not uniformly represent the upstream catchment area. We observe strong impacts on Be-10 concentrations from local, nonfluvial geomorphic processes, such as glaciation and landsliding coinciding with areas of peak rainfall rates, pointing toward climatic modulation of predominantly tectonically driven denudation rates.}, language = {en} }