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Subfossil Cladocera were sampled and examined from the surface sediments of 35 thermokarst lakes along a temperature gradient crossing the tree line in the Anabar-river basin in northwestern Yakutia, northeastern Siberia. The lakes were distributed through three environmental zones: typical tundra, southern tundra and forest tundra. All lakes were situated within the continuous permafrost zone. Our investigation showed that the cladoceran communities in the lakes of the Anabar region are diverse and abundant, as reflected by taxonomic richness, and high diversity and evenness indices (H = 1.89 +/- A 0.51; I = 0.8 +/- A 0.18). CONISS cluster analysis indicated that the cladoceran communities in the three ecological zones (typical tundra, southern tundra and forest-tundra) differed in their taxonomic composition and structure. Differences in the cladoceran assemblages were related to limnological features and geographical position, vegetation type, climate and water chemistry. The constrained redundancy analysis indicated that T-July, water depth and both sulphate (SO4 (2-)) and silica (Si4+) concentrations significantly (p a parts per thousand currency sign 0.05) explained variance in the cladoceran assemblage. T-July featured the highest percentage (17.4 %) of explained variance in the distribution of subfossil Cladocera. One of the most significant changes in the structure of the cladoceran communities in the investigated transect was the replacement of closely related species along the latitudinal and vegetation gradient. The results demonstrate the potential for a regional cladoceran-based temperature model for the Arctic regions of Russia, and for and Yakutia in particular.
Many Mediterranean drylands are characterized by strong erosion in headwater catchments, where connectivity processes play an important role in the redistribution of water and sediments. Sediment connectivity describes the ease with which sediment can move through a catchment. The spatial and temporal characterization of connectivity patterns in a catchment enables the estimation of sediment contribution and transfer paths. Apart from topography, vegetation cover is one of the main factors driving sediment connectivity. This is particularly true for the patchy vegetation cover typical of many dryland environments. Several connectivity measures have been developed in the last few years. At the same time, advances in remote sensing have enabled an improved catchment-wide estimation of ground cover at the subpixel level using hyperspectral imagery.
The objective of this study was to assess the sediment connectivity for two adjacent subcatchments (similar to 70 km(2)) of the Isabena River in the Spanish Pyrenees in contrasting seasons using a quantitative connectivity index based on fractional vegetation cover and topography data. The fractional cover of green vegetation, non-photosynthetic vegetation, bare soil and rock were derived by applying a multiple endmember spectral mixture analysis approach to the hyperspectral image data. Sediment connectivity was mapped using the index of connectivity, in which the effect of land cover on runoff and sediment fluxes is expressed by a spatially distributed weighting factor. In this study, the cover and management factor (C factor) of the Revised Universal Soil Loss Equation (RUSLE) was used as a weighting factor. Bi-temporal C factor maps were derived by linking the spatially explicit fractional ground cover and vegetation height obtained from the airborne data to the variables of the RUSLE subfactors.
The resulting connectivity maps show that areas behave very differently with regard to connectivity, depending on the land cover and on the spatial distribution of vegetation abundances and topographic barriers. Most parts of the catchment show higher connectivity values in August as compared to April. The two subcatchments show a slightly different connectivity behaviour that reflects the different land cover proportions and their spatial configuration.
The connectivity estimation can support a better understanding of processes controlling the redistribution of water and sediments from the hillslopes to the channel network at a scale appropriate for land management. It allows hot spot areas of erosion to be identified and the effects of erosion control measures, as well as different land management scenarios, to be studied.
Trees and shrubs in tropical Africa use the C-3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C-4 cycle. Leaf-wax lipids from sedimentary archives such as the long-chain n-alkanes (e.g., n-C-27 to n-C-33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n-alkane delta C-13 values are often used to reconstruct past C-3/C-4 composition of vegetation, assuming that the relative proportions of C-3 and C-4 leaf waxes reflect the relative proportions of C-3 and C-4 plants. We have compared the delta C-13 values of n-alkanes from modern C-3 and C-4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C-3 vegetation cover (f(C3)) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C-3-dominated rain forest by C-4-dominated savanna. The C-3 plants analysed were characterised by substantially higher abundances of n-C-29 alkanes and by substantially lower abundances of n-C-33 alkanes than the C-4 plants. Furthermore, the sedimentary delta C-13 values of n-C-29 and n-C-31 alkanes from recent lake sediments in Cameroon (-37.4%) to 26.5%) were generally within the range of delta C-13 values for C-3 plants, even when from sites where C-4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C-3 and C-4 vegetation cover when using the delta C-13 values of sedimentary n-alkanes, overestimating the proportion of C-3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C-3 and C-4 plants. We therefore tested a set of non-linear binary mixing models using delta C-13 values from both C-3 and C-4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the f(C3) values as the minimum and maximum delta C-13 values are approached, and a hyperbolic function that takes into account the differences between C-3 and C-4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed delta C-13 values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert delta C-13 measurements on sedimentary n-alkanes directly into reconstructions of C-3 vegetation cover. (C) 2014 Elsevier Ltd. All rights reserved.
Kinematic interaction of faults is an important issue for detailed seismic hazard assessments in seismically active regions. The Eastern Mosha Fault (EMF) and the North Tehran Fault (NTF) are two major active faults of the southern central Alborz mountains, located in proximity of Tehran (population similar to 9 million). We used field, geomorphological and paleoseismological data to explore the kinematic transition between the faults, and compare their short-term and long-term history of activity. We introduce the Niknamdeh segment of the NTF along which the strike-slip kinematics of EMF is transferred onto the NTF, and which is also responsible for the development of a pull-apart basin between the eastern segments of the NTF. The Ira trench site at the linkage zone between the two faults reveals the history of interaction between rock avalanches, active faulting and sag-pond development. The kinematic continuity between the EMF and NTF requires updating of seismic hazard models for the NTF, the most active fault adjacent to the Tehran Metropolis. Study of offsets of large-scale morphological features along the EMF, and comparison with estimated slip rates along the fault indicates that the EMF has started its left-lateral kinematics between 3.2 and 4.7 Ma. According to our paleoseismological data and the morphology of the nearby EMF and NTF, we suggest minimum and maximum values of about 1.8 and 3.0 mm/year for the left-lateral kinematics on the two faults in their linkage zone, averaged over Holocene time scales. Our study provides a partial interpretation, based on available data, for the fault activity in northeastern Tehran region, which may be completed with studies of other active faults of the region to evaluate a more realistic seismic hazard analysis for this heavily populated major city. (C) 2014 Elsevier B.V. All rights reserved.
Members of the acI lineage of Actinobacteria are the most abundant microorganisms in most freshwater lakes; however, our understanding of the keys to their success and their role in carbon and nutrient cycling in freshwater systems has been hampered by the lack of pure cultures and genomes. We obtained draft genome assemblies from 11 single cells representing three acI tribes (acI-A1, acI-A7, acI-B1) from four temperate lakes in the United States and Europe. Comparative analysis of acI SAGs and other available freshwater bacterial genomes showed that acI has more gene content directed toward carbohydrate acquisition as compared to Polynucleobacter and LD12 Alphaproteobacteria, which seem to specialize more on carboxylic acids. The acI genomes contain actinorhodopsin as well as some genes involved in anaplerotic carbon fixation indicating the capacity to supplement their known heterotrophic lifestyle. Genome-level differences between the acI-A and acI-B clades suggest specialization at the clade level for carbon substrate acquisition. Overall, the acI genomes appear to be highly streamlined versions of Actinobacteria that include some genes allowing it to take advantage of sunlight and N-rich organic compounds such as polyamines, di-and oligopeptides, branched-chain amino acids and cyanophycin. This work significantly expands the known metabolic potential of the cosmopolitan freshwater acI lineage and its ecological and genetic traits.
Inferring a ground-motion prediction equation (GMPE) for a region in which only a small number of seismic events has been observed is a challenging task. A response to this data scarcity is to utilise data from other regions in the hope that there exist common patterns in the generation of ground motion that can contribute to the development of a GMPE for the region in question. This is not an unreasonable course of action since we expect regional GMPEs to be related to each other. In this work we model this relatedness by assuming that the regional GMPEs occupy a common low-dimensional manifold in the space of all possible GMPEs. As a consequence, the GMPEs are fitted in a joint manner and not independent of each other, borrowing predictive strength from each other's regional datasets. Experimentation on a real dataset shows that the manifold assumption displays better predictive performance over fitting regional GMPEs independent of each other. (C) 2014 Elsevier Ltd. All rights reserved.
The 2002 M-w 7.9 Denali Fault earthquake, Alaska, provides an unparalleled opportunity to investigate in quantitative detail the regional hillslope mass-wasting response to strong seismic shaking in glacierized terrain. We present the first detailed inventory of similar to 1580 coseismic slope failures, out of which some 20% occurred above large valley glaciers, based on mapping from multi-temporal remote sensing data. We find that the Denali earthquake produced at least one order of magnitude fewer landslides in a much narrower corridor along the fault ruptures than empirical predictions for an M 8 earthquake would suggest, despite the availability of sufficiently steep and dissected mountainous topography prone to frequent slope failure. In order to explore potential controls on the reduced extent of regional coseismic landsliding we compare our data with inventories that we compiled for two recent earthquakes in periglacial and formerly glaciated terrain, i.e. at Yushu, Tibet (M-w 6.9, 2010), and Aysen Fjord, Chile (2007 M-w 6.2). Fault movement during these events was, similarly to that of the Denali earthquake, dominated by strike-slip offsets along near-vertical faults. Our comparison returns very similar coseismic landslide patterns that are consistent with the idea that fault type, geometry, and dynamic rupture process rather than widespread glacier cover were among the first-order controls on regional hillslope erosional response in these earthquakes. We conclude that estimating the amount of coseismic hillslope sediment input to the sediment cascade from earthquake magnitude alone remains highly problematic, particularly if glacierized terrain is involved. (C) 2014 Elsevier Ltd. All rights reserved.
Sedimentary proxy records constitute a significant portion of the recorded evidence that allows us to investigate paleoclimatic conditions and variability. However, uncertainties in the dating of proxy archives limit our ability to fix the timing of past events and interpret proxy record intercomparisons. While there are various age-modeling approaches to improve the estimation of the age-depth relations of archives, relatively little focus has been placed on the propagation of the age (and radiocarbon calibration) uncertainties into the final proxy record.
We present a generic Bayesian framework to estimate proxy records along with their associated uncertainty, starting with the radiometric age-depth and proxy-depth measurements, and a radiometric calibration curve if required. We provide analytical expressions for the posterior proxy probability distributions at any given calendar age, from which the expected proxy values and their uncertainty can be estimated. We illustrate our method using two synthetic data sets and then use it to construct the proxy records for groundwater inflow and surface erosion from Lonar lake in central India.
Our analysis reveals interrelations between the uncertainty of the proxy record over time and the variance of proxies along the depth of the archive. For the Lonar lake proxies, we show that, rather than the age uncertainties, it is the proxy variance combined with calibration uncertainty that accounts for most of the final uncertainty. We represent the proxy records as probability distributions on a precise, error-free timescale that makes further time series analyses and intercomparisons of proxies relatively simple and clear. Our approach provides a coherent understanding of age uncertainties within sedimentary proxy records that involve radiometric dating. It can be potentially used within existing age modeling structures to bring forth a reliable and consistent framework for proxy record estimation.
Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterised by low signal-to-noise ratio, such methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this thesis I developed a picking free automated method based on the Short-Term-Average/Long-Term-Average (STA/LTA) traces at different stations as observed data. I used the STA/LTA of several characteristic functions in order to increase the sensitiveness to the P wave and the S waves. For the P phases we use the STA/LTA traces of the vertical energy function, while for the S phases, we use the STA/LTA traces of the horizontal energy trace and then a more optimized characteristic function which is obtained using the principal component analysis technique. The orientation of the horizontal components can be retrieved by robust and linear approach of waveform comparison between stations within a network using seismic sources outside the network (chapter 2). To locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a three-dimensional grid we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial and temporal coordinates of the seismic event. Location uncertainties are then estimated by perturbing the STA/LTA parameters (i.e the length of both long and short time windows) and relocating each event several times. In order to test the location method I firstly applied it to a set of 200 synthetic events. Then we applied it to two different real datasets. A first one related to mining induced microseismicity in a coal mine in the northern Germany (chapter 3). In this case we successfully located 391 microseismic event with magnitude range between 0.5 and 2.0 Ml. To further validate the location method I compared the retrieved locations with those obtained by manual picking procedure. The second dataset consist in a pilot application performed in the Campania-Lucania region (southern Italy) using a 33 stations seismic network (Irpinia Seismic Network) with an aperture of about 150 km (chapter 4). We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < Ml < 2.7. A subset of these locations were compared with accurate locations retrieved by a manual location procedure based on the use of a double difference technique. In both cases results indicate good agreement with manual locations. Moreover, the waveform stacking location method results noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals.
Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterized by low signal-to-noise ratio, automated location methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this paper, we propose a picking free earthquake location method based on the use of the short-term-average/long-term-average (STA/LTA) traces at different stations as observed data. For the P phases, we use the STA/LTA traces of the vertical energy function, whereas for the S phases, we use the STA/LTA traces of a second characteristic function, which is obtained using the principal component analysis technique. In order to locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a 3-D grid, we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event. A pilot application was performed in the Campania-Lucania region (southern Italy) using a seismic network (Irpinia Seismic Network) with an aperture of about 150 km. We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < M-L < 2.7. A subset of these locations were compared with accurate manual locations refined by using a double-difference technique. Our results indicate a good agreement with manual locations. Moreover, our method is noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals.
In situ UV laser spot Ar-40/Ar-39 analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent Ar-40/Ar-39 ages of phengite cores span a range of similar to 20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 and 77-74 Ma, respectively. The younger cores have on average lower B contents (similar to 36 mu g/g) than the older ones (similar to 43-48 mu g/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable delta B-11 values (-18 parts per thousand to -10 parts per thousand), indicating the lack of km scale B homogenization during HP crystallization.
Overprinted phengite rims in the weakly deformed samples generally yield younger apparent Ar-40/Ar-39 ages than the respective cores. They also show variable effects of heterogeneous excess 40 Ar incorporation and Ar loss. One acceptable inverse isochron age of 77.1 +/- 1.1 Ma for rims surrounding older cores (82.6 +/- 0.6 Ma) overlaps with the second period of core crystallization. Compared to the phengite cores, all rims have lower B and Li abundances but similar delta B-11 values (-15 parts per thousand to -9 parts per thousand), reflecting internal redistribution of B and Li and internal fluid buffering of the B isotopic composition during rim growth. The combined observation of younger Ar-40/Ar-39 ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains.
Strongly deformed samples from a several km long, blueschist-facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent Ar-40/Ar-39 ages (64.7-68.8 Ma), yielding an inverse isochron age of 65.0 +/- 3.0 Ma. Almost complete B and Li removal in mylonitic phengite is due to leaching into a fluid. The B isotopic composition is significantly heavier than in phengites from the weakly deformed samples, indicating an external control by a high-delta B-11 fluid (delta B-11 = + 7 +/- 4 parts per thousand). We interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone. This event also caused partial resetting of the Ar isotope system and further B loss in more permeable rocks of the adjacent unit. We conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of Ar-40/Ar-39 data in partially metasomatized rocks.
Velocity models are essential to process two-and three-dimensional ground-penetrating radar (GPR) data. Furthermore, velocity information aids the interpretation of such data sets because velocity variations reflect important material properties such as water content. In many GPR applications, common midpoint (CMP) surveys are routinely collected to determine one-dimensional velocity models at selected locations. To analyse CMP data gathers, spectral velocity analyses relying on the normal-moveout (NMO) model are commonly employed. Using Dix's formula, the derived NMO velocities can be further converted to interval velocities which are needed for processing and interpretation. Because of the inherent assumptions and limitations of such approaches, we investigate and propose an alternative procedure based on the global inversion of reflection travel-times. We use a finite-difference solver of the Eikonal equation to accurately solve the forward problem in combination with particle swarm optimization (PSO) to find one-dimensional GPR velocity models explaining our data. Because PSO is a robust and efficient global optimization tool, our inversion approach includes generating an ensemble of representative solutions that allows us to analyse uncertainties in the model space. Using synthetic data examples, we test and evaluate our inversion approach to analyse CMP data collected across typical near-surface environments. Application to a field data set recorded at a well-constrained test site including a comparison to independent borehole and direct-push data, further illustrates the potential of the proposed approach, which includes a straightforward and understandable appraisal of non-uniqueness and uncertainty issues, respectively. We conclude that our methodology is a feasible and powerful tool to analyse GPR CMP data and allows practitioners and researchers to evaluate the reliability of CMP derived velocity models.
Marine seismology usually relies on temporary deployments of stand alone seismic ocean bottom stations (OBS), which are initialized and synchronized on ship before deployment and re-synchronized and stopped on ship after recovery several months later. In between, the recorder clocks may drift and float at unknown rates. If the clock drifts are large or not linear and cannot be corrected for, seismological applications will be limited to methods not requiring precise common timing. Therefore, for example, array seismological methods, which need very accurate timing between individual stations, would not be applicable for such deployments.
We use an OBS test-array of 12 stations and 75 km aperture, deployed for 10 months in the deep sea (4.5-5.5 km) of the mid-eastern Atlantic. The experiment was designed to analyse the potential of broad-band array seismology at the seafloor. After recovery, we identified some stations which either show unusual large clock drifts and/or static time offsets by having a large difference between the internal clock and the GPS-signal (skew).
We test the approach of ambient noise cross-correlation to synchronize clocks of a deep water OBS array with km-scale interstation distances. We show that small drift rates and static time offsets can be resolved on vertical components with a standard technique. Larger clock drifts (several seconds per day) can only be accurately recovered if time windows of one input trace are shifted according to the expected drift between a station pair before the cross-correlation. We validate that the drifts extracted from the seismometer data are linear to first order. The same is valid for most of the hydrophones. Moreover, we were able to determine the clock drift at a station where no skew could be measured. Furthermore, we find that instable apparent drift rates at some hydrophones, which are uncorrelated to the seismometer drift recorded at the same digitizer, indicate a malfunction of the hydrophone.
We examine the use of ambient noise cross-correlation tomography for shallow site characterization using a modified two-step approach. Initially, we extract Rayleigh wave traveltimes from correlation traces of vertical component seismic recordings from a local network installed in Mygdonia basin, northern Greece. The obtained Rayleigh wave traveltimes show significant spatial variability, as well as distance and frequency dependence due to the 3-D structure of the area, dispersion, and anelastic attenuation effects. The traveltime data sets are inverted through a surface wave tomography approach to determine group velocity maps for each frequency. The proposed tomographic inversion involves the use of approximate Fresnel volumes and interfrequency smoothing constraints to stabilize the results. In the last step, we determine a final 3-D velocity model using a node-based Monte Carlo 1-D dispersion curve inversion. The reliability of the final 3-D velocity model is examined by spatial and depth resolution analysis, as well as by inversion for different model parameterizations. The obtained results are in very good agreement with previous findings from seismic and other geophysical methods. The new 3-D VS model provides additional structural constraints for the shallow sediments and bedrock structure of the northern Mygdonia basin up to the depth of similar to 200-250 m. Present work results suggest that the migration of ambient tomography techniques from large scales (tens or hundreds of km) to local scales (few hundred meters) is possible but cannot be used as a black box technique for 3-D modeling and detailed geotechnical site characterization.
The ratio of unsupported protactinium-231 to thorium-230 in marine sediments, (Pa/Th)(xs), is potentially sensitive to several processes of oceanographic and climatological interest: deep ocean circulation, marine biological productivity (as it relates to total particle flux) and particle composition (specifically, biogenic opal and authigenic Mn). In order to attribute variations in (Pa/Th)(xs) observed in sediment records to changes in specific processes through time, a better understanding of the chemical cycling of these elements in the modern ocean is necessary. To this end, a survey was undertaken of (Pa/Th)(xs) in surface sediments from the subarctic Pacific (SO202-INOPEX expedition) in combination with a Pacific-wide compilation of published data. Throughout the Pacific, (Pa/Th)(xs) is robustly correlated with the opal content of sediments. In the North and equatorial Pacific, simultaneous positive correlations with productivity indicators suggest that boundary scavenging and opal scavenging combine to enhance the removal of Pa in the eastern equatorial Pacific and subarctic Pacific. Deep ocean water mass ageing (>3.5 km) associated with the Pacific overturning appears to play a secondary role in determining the basin scale distribution of (Pa/Th)(xs). A basin-wide extrapolation of Pa removal is performed which suggests that the Pacific Pa budget is nearly in balance. We hypothesize that through time (Pa/Th)(xs) distributions in the Pacific could define the evolving boundaries of contrasting biogeographic provinces in the North Pacific, while the influence of hydrothermal scavenging of Pa potentially confounds this approach in the South Pacific.
Soil in a changing world is subject to both anthropogenic and environmental stresses. Soil monitoring is essential to assess the magnitude of changes in soil variables and how they affect ecosystem processes and human livelihoods. However, we cannot always be sure which sampling design is best for a given monitoring task. We employed a rotational stratified simple random sampling (rotStRS) for the estimation of temporal changes in the spatial mean of saturated hydraulic conductivity (K-s) at three sites in central Panama in 2009, 2010 and 2011. To assess this design's efficiency we compared the resulting estimates of the spatial mean and variance for 2009 with those gained from stratified simple random sampling (StRS), which was effectively the data obtained on the first sampling time, and with an equivalent unexecuted simple random sampling (SRS). The poor performance of geometrical stratification and the weak predictive relationship between measurements of successive years yielded no advantage of sampling designs more complex than SRS. The failure of stratification may be attributed to the small large-scale variability of K-s. Revisiting previously sampled locations was not beneficial because of the large small-scale variability in combination with destructive sampling, resulting in poor consistency between revisited samples. We conclude that for our K-s monitoring scheme, repeated SRS is equally effective as rotStRS. Some problems of small-scale variability might be overcome by collecting several samples at close range to reduce the effect of small-scale variation. Finally, we give recommendations on the key factors to consider when deciding whether to use stratification and rotation in a soil monitoring scheme.
The 933 km(2) Bengue catchment in northeastern Brazil is characterized by distinct rainy and dry seasons. Precipitation is stored in variously sized reservoirs, which is essential for the local population. In this study, we used TerraSAR-X SM(HH) data for an one-year monitoring of seasonal changes in the reservoir areas from July 2011 to July 2012. The monitoring was based on acquisitions in the ascending pass direction, complemented by occasional descending-pass images. To detect water surface areas, a histogram analysis followed by a global threshold classification was performed, and the results were validated using in situ GPS data. Distinguishing between small reservoirs and similar looking dark areas was difficult. Therefore, we tested several approaches for identifying misclassified areas. An analysis of the surface area dynamics of the reservoirs indicated high spatial and temporal heterogeneities and a large decrease in the total water surface area of the reservoirs in the catchment by approximately 30% within one year.
In a study from 2008, Lariviere and colleagues showed, for the field of natural sciences and engineering, that the median age of cited references is increasing over time. This result was considered counterintuitive: with the advent of electronic search engines, online journal issues and open access publications, one could have expected that cited literature is becoming younger. That study has motivated us to take a closer look at the changes in the age distribution of references that have been cited in water resources journals since 1965. Not only could we confirm the findings of Lariviere and colleagues. We were also able to show that the aging is mainly happening in the oldest 10-25% of an average reference list. This is consistent with our analysis of top-cited papers in the field of water resources. Rankings based on total citations since 1965 consistently show the dominance of old literature, including text books and research papers in equal shares. For most top-cited old-timers, citations are still growing exponentially. There is strong evidence that most citations are attracted by publications that introduced methods which meanwhile belong to the standard toolset of researchers and practitioners in the field of water resources. Although we think that this trend should not be overinterpreted as a sign of stagnancy, there might be cause for concern regarding how authors select their references. We question the increasing citation of textbook knowledge as it holds the risk that reference lists become overcrowded, and that the readability of papers deteriorates.