TY - JOUR A1 - Forbrich, Inke A1 - Kutzbach, Lars A1 - Hormann, Annabell A1 - Wilmking, Martin T1 - A comparison of linear and exponential regression for estimating diffusive CH4 fluxes by closed-chambers in peatlands N2 - The closed-chamber method is the most common approach to determine CH4 fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non- linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge- covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-overtime curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH4 concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/00380717 U6 - https://doi.org/10.1016/j.soilbio.2009.12.004 SN - 0038-0717 ER - TY - JOUR A1 - Wilke, Max A1 - Appel, Karen A1 - Vincze, Laszlo A1 - Schmidt, Christian A1 - Borchert, Manuela A1 - Pascarelli, Sakura T1 - A confocal set-up for micro-XRF and XAFS experiments using diamond-anvil cells N2 - A confocal set-up is presented that improves micro-XRF and XAFS experiment with high-pressure e diamond-anvil cells (DACs) In this experiment a probing volume is defined by the focus of the incoming synchrotron radiation beam and that of a polycapillary X-ray half-lens with a very long working distance, which is placed in front of the fluorescence detector This set-up enhances the quality of the fluorescence and XAFS spectra, and thus the sensitivity for detecting elements at low concentrations. It efficiently suppresses signal from outside the sample chamber, which stems from elastic and inelastic scattering of the incoming beam by the diamond anvils as well as from excitation of fluorescence from the body of the DAC Y1 - 2010 UR - http://journals.iucr.org/s/journalhomepage.html U6 - https://doi.org/10.1107/S0909049510023654 SN - 0909-0495 ER - TY - JOUR A1 - Herzschuh, Ulrike A1 - Birks, H. John B. A1 - Mischke, Steffen A1 - Zhang, Chengjun A1 - Böhner, Jürgen T1 - A modern pollen-climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains N2 - Aim: Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen-climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location: The Tibetan Plateau, between 30 and 40 degrees N and 87 and 103 degrees E. Methods: We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (P-ann; 31-1022 mm), mean annual temperature (T-ann; -6.5 to 1 degrees C), and mean July temperature (T-July; 2.6-19.7 degrees C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for P-ann, T-ann and T-July were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results: The root mean square errors of prediction (RMSEP) were 104 mm (P-ann), 1.18 degrees C (T-ann) and 1.17 degrees C (T-July). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (P-ann), 15.7% (T-ann) and 11.9% (T-July). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern P-ann 480 mm; T-ann-1 degrees C). T-ann and P-ann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 degrees C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 degrees C greater than present). Main conclusions: The modern pollen-climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance. Y1 - 2010 UR - http://www3.interscience.wiley.com/cgi-bin/issn?DESCRIPTOR=PRINTISSN&VALUE=0305-0270 U6 - https://doi.org/10.1111/j.1365-2699.2009.02245.x SN - 0305-0270 ER - TY - JOUR A1 - Gräff, Thomas A1 - Zehe, Erwin A1 - Schläger, Stefan A1 - Morgner, Markus A1 - Bauer, Andreas A1 - Becker, Rolf A1 - Creutzfeldt, Benjamin A1 - Bronstert, Axel T1 - A quality assessment of spatial TDR soil moisture measurements in homogenous and heterogeneous media with laboratory experiments N2 - Investigation of transient soil moisture profiles yields valuable information of near- surface processes. A recently developed reconstruction algorithm based on the telegraph equation allows the inverse estimation of soil moisture profiles along coated, three rod TDR probes. Laboratory experiments were carried out to prove the results of the inversion and to understand the influence of probe rod deformation and solid objects close to the probe in heterogonous media. Differences in rod geometry can lead to serious misinterpretations in the soil moisture profile but have small influence on the average soil moisture along the probe. Solids in the integration volume have almost no effect on average soil moisture but result in locally slightly decreased moisture values. Inverted profiles obtained in a loamy soil with a clay content of about 16% were in good agreement with independent measurements. Y1 - 2010 UR - http://www.hydrol-earth-syst-sci-discuss.net/volumes_and_issues.html U6 - https://doi.org/10.5194/hessd-7-269-2010 SN - 1812-2108 ER - TY - JOUR A1 - Gräff, Thomas A1 - Zehe, Erwin A1 - Schlaeger, Stefan A1 - Morgner, Markus A1 - Bauer, Andreas A1 - Becker, Rolf A1 - Creutzfeldt, Benjamin A1 - Bronstert, Axel T1 - A quality assessment of Spatial TDR soil moisture measurements in homogenous and heterogeneous media with laboratory experiments N2 - Investigation of transient soil moisture profiles yields valuable information of near- surface processes. A recently developed reconstruction algorithm based on the telegraph equation allows the inverse estimation of soil moisture profiles along coated, three rod TDR probes. Laboratory experiments were carried out to prove the results of the inversion and to understand the influence of probe rod deformation and solid objects close to the probe in heterogeneous media. Differences in rod geometry can lead to serious misinterpretations in the soil moisture profile, but have small influence on the average soil moisture along the probe. Solids in the integration volume have almost no effect on average soil moisture, but result in locally slightly decreased moisture values. Inverted profiles obtained in a loamy soil with a clay content of about 16% were in good agreement with independent measurements. Y1 - 2010 UR - http://www.copernicus.org/EGU/hess/hess.html U6 - https://doi.org/10.5194/hess-14-1007-2010 SN - 1027-5606 ER - TY - JOUR A1 - Köhler, Birgit A1 - Zehe, Erwin A1 - Corre, Marife D. A1 - Veldkamp, Edzo T1 - An inverse analysis reveals limitations of the soil-CO2 profile method to calculate CO2 production and efflux for well-structured soils N2 - Soil respiration is the second largest flux in the global carbon cycle, yet the underlying below-ground process, carbon dioxide (CO2) production, is not well understood because it can not be measured in the field. CO2 production has frequently been calculated from the vertical CO2 diffusive flux divergence, known as "soil-CO2 profile method". This relatively simple model requires knowledge of soil CO2 concentration profiles and soil diffusive properties. Application of the method for a tropical lowland forest soil in Panama gave inconsistent results when using diffusion coefficients (D) calculated based on relationships with soil porosity and moisture ("physically modeled" D). Our objective was to investigate whether these inconsistencies were related to (1) the applied interpolation and solution methods and/or (2) uncertainties in the physically modeled profile of D. First, we show that the calculated CO2 production strongly depends on the function used to interpolate between measured CO2 concentrations. Secondly, using an inverse analysis of the soil-CO2 profile method, we deduce which D would be required to explain the observed CO2 concentrations, assuming the model perception is valid. In the top soil, this inversely modeled D closely resembled the physically modeled D. In the deep soil, however, the inversely modeled D increased sharply while the physically modeled D did not. When imposing a constraint during the fit parameter optimization, a solution could be found where this deviation between the physically and inversely modeled D disappeared. A radon (Rn) mass balance model, in which diffusion was calculated based on the physically modeled or constrained inversely modeled D, simulated observed Rn profiles reasonably well. However, the CO2 concentrations which corresponded to the constrained inversely modeled D were too small compared to the measurements. We suggest that, in well-structured soils, a missing description of steady state CO2 exchange fluxes across water-filled pores causes the soil-CO2 profile method to fail. These fluxes are driven by the different diffusivities in inter- vs. intra-aggregate pores which create permanent CO2 gradients if separated by a "diffusive water barrier". These results corroborate other studies which have shown that the theory to treat gas diffusion as homogeneous process, a precondition for use of the soil-CO2 profile method, is inaccurate for pore networks which exhibit spatial separation between CO2 production and diffusion out of the soil. Y1 - 2010 UR - http://www.copernicus.org/EGU/bg/bg.html U6 - https://doi.org/10.5194/bg-7-2311-2010 SN - 1726-4170 ER - TY - JOUR A1 - Zimmermann, Beate A1 - Papritz, Andreas A1 - Elsenbeer, Helmut T1 - Asymmetric response to disturbance and recovery : Changes of soil permeability under forest-pasture-forest transitions N2 - In the humid tropics, continuing high deforestation rates are seen alongside an increasing expansion of secondary forests. In order to understand and model the consequences of these dynamic land-use changes for regional water cycles, the response of soil hydraulic properties to forest disturbance and recovery has to be quantified.At a site in the Brazilian Amazonia, we annually monitored soil infiltrability and saturated hydraulic conductivity (K-s) at 12.5, 20 cm, and 50 cm soil depth after manual forest conversion to pasture (year zero to four after pasture establishment), and during secondary succession after pasture abandonment (year zero to seven after pasture abandonment). We evaluated the hydrological consequences of the detected changes by comparing the soil hydraulic properties with site-specific rainfall intensities and hydrometric observations. Within one year after grazing started, infiltrability and K-s at 12.5 and 20 cm depth decreased by up to one order of magnitude to levels which are typical for 20-year-old pasture. In the three subsequent monitoring years, infiltrability and K-s remained stable. Land use did not impact on subsoil permeability. Whereas infiltrability values are large enough to allow all rainwater to infiltrate even after the conversion, the sudden decline of near-surface K-s is of hydrological relevance as perched water tables and overland flow occur more often on pastures than in forests at our study site. After pasture abandonment and during secondary succession, seven years of recovery did not suffice to significantly increase infiltrability and K-s at 12.5 depth although a slight recovery is obvious. At 20 cm soil depth, we detected a positive linear increase within the seven-year time frame but annual means did not differ significantly. Although more than a doubling of infiltrability and K-s is still required to achieve pre-disturbance levels, which will presumably take more than a decade, the observed slight increases of K-s might already decrease the probability of perched water table generation and overland flow development well before complete recovery. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/00167061 U6 - https://doi.org/10.1016/j.geoderma.2010.07.013 SN - 0016-7061 ER - TY - JOUR A1 - Clarke, Brian A. A1 - Burbank, Douglas W. T1 - Bedrock fracturing, threshold hillslopes, and limits to the magnitude of bedrock landslides N2 - Bedrock fracturing and rock strength are widely believed to influence landscape morphology and erosional resistance. Yet, understanding of the quantitative relationship between rock-mass strength and landscape evolution remains limited. Here we present a new application of seismic refraction surveys that uses variations in seismic velocity to interpret differences in bedrock fracture density with depth. We use a comparative study of Fiordland and the western Southern Alps of New Zealand to examine how differences in rock type and bedrock fracturing influence landscape morphology and landslide response to rock uplift. In both regions, slopes appear invariant with differential rock-uplift rates and slope distributions reveal modal hillslope angles of similar to 32 degrees. The majority of landslides initiate on slopes steeper than the modal hillslope angle, however, landslide magnitude-frequency distributions reveal order-of-magnitude differences between the regions, with Fiordland experiencing considerably smaller and less frequent landsliding events. Landslide-driven denudation rates of similar to 9 mm/yr in the western Southern Alps and between similar to 0.1 and 0.3 mm/yr in Fiordland approximate estimates of long-term rock-uplift rates for each region. The invariance of hillslope angles, near-normal slope distributions, predominance of landslide initiation on slopes steeper than modal values, and the apparent balance between rates of uplift and landslide-driven erosion suggest that hillslopes in both regions are at threshold angles. Their similar modal slopes further suggest that both ranges are characterized by equivalent rock-mass strength, despite striking differences in lithology. Additionally, our seismic analysis reveals nearly identical surface p-wave velocities. The unexpected equivalence of both modal slopes and surface velocities between these lithologically distinct ranges is attributed to contrasting degrees of surface fracturing that have differentially affected the intact rock properties, such that they now yield equivalent surface velocities and hillslope-scale strengths. Given that surface fractures help regulate threshold angles by modulating hillslope strength; we propose that shallow seismic velocities may provide a quantitative proxy for rock-mass strength. We define two contrasting fracture and landsliding environments. In Fiordland, dense geomorphic fracturing that is focused within the shallow subsurface appears to limit the depth and magnitude of most bedrock landslides. Conversely, in the western Southern Alps, tectonic forces produce pervasive fracturing with depth that results in larger, and deeper landslides. Our data suggest that bedrock fracturing at the Earth's surface modulates threshold hillslope angles, whereas the depth of bedrock fracturing influences the magnitude and frequency of landslide response to tectonic rock uplift. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/0012821X U6 - https://doi.org/10.1016/j.epsl.2010.07.011 SN - 0012-821X ER - TY - JOUR A1 - Siliverstovs, Boriss A1 - Ötsch, Rainald A1 - Kemfert, Claudia A1 - Jaeger, Carlo A1 - Haas, Armin A1 - Kremers, Hans T1 - Climate change and modelling of extreme temperatures in Switzerland N2 - This study models maximum temperatures in Switzerland monitored in twelve locations using the generalised extreme value (GEV) distribution. The parameters of the GEV distribution are determined within a Bayesian framework. We find that the parameters of the underlying distribution underwent a substantial change in the beginning of the 1980s. This change is characterised by an increase both in the level and the variability. We assess the likelihood of the heat wave of the summer 2003 using the fitted GEV distribution by accounting for the presence of a structural break. The estimation results do suggest that the heat wave of 2003 is not that statistically improbable if an appropriate methodology is used for dealing with nonstationarity. Y1 - 2010 UR - http://www.springerlink.com/content/103283 U6 - https://doi.org/10.1007/s00477-009-0321-3 SN - 1436-3240 ER - TY - JOUR A1 - Dahm, Torsten A1 - Kuehn, Daniela A1 - Ohrnberger, Matthias A1 - Kroeger, Jens A1 - Wiederhold, Helga A1 - Reuther, Claus-Dieter A1 - Dehghani, Ali A1 - Scherbaum, Frank T1 - Combining geophysical data sets to study the dynamics of shallow evaporites in urban environments : application to Hamburg, Germany N2 - Shallowly situated evaporites in built-up areas are of relevance for urban and cultural development and hydrological regulation. The hazard of sinkholes, subrosion depressions and gypsum karst is often difficult to evaluate and may quickly change with anthropogenic influence. The geophysical exploration of evaporites in metropolitan areas is often not feasible with active industrial techniques. We collect and combine different passive geophysical data as microgravity, ambient vibrations, deformation and hydrological information to study the roof morphology of shallow evaporites beneath Hamburg, Northern Germany. The application of a novel gravity inversion technique leads to a 3-D depth model of the salt diapir under study. We compare the gravity-based depth model to pseudo-depths from H/V measurements and depth estimates from small-scale seismological array data. While the general range and trend of the diapir roof is consistent, a few anomalous regions are identified where H/V pseudo-depths indicate shallower structures not observed in gravity or array data. These are interpreted by shallow residual caprock floaters and zones of increased porosity. The shallow salt structure clearly correlates with a relative subsidence in the order of 2 mm yr(-1). The combined interpretation of roof morphology, yearly subsidence rates, chemical analyses of groundwater and of hydraulic head in aquifers indicates that the salt diapir beneath Hamburg is subject to significant ongoing dissolution that may possibly affect subrosion depressions, sinkhole distribution and land usage. The combined analysis of passive geophysical data may be exemplary for the study of shallow evaporites beneath other urban areas. Y1 - 2010 UR - http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-246X U6 - https://doi.org/10.1111/j.1365-246X.2010.04521.x SN - 0956-540X ER -