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  • 1
    Language: English
    In: Journal of Hydrology, October 2015, Vol.529, pp.1754-1767
    Description: Soil moisture plays a key role in the water and energy balance in soil, vegetation and atmosphere systems. According to Wood et al. (2011) there is a grand need to increase global-scale hyper-resolution water–energy–biogeochemistry land surface modelling capabilities. These modelling capabilities should also recognize epistemic uncertainties, as well as the nonlinearity and hysteresis in its dynamics. Unfortunately, it is not clear how to parameterize hydrological processes as a function of scale, and how to test deterministic models with regard to epistemic uncertainties. In this study, high resolution long-term simulations were conducted in the highly instrumented TERENO hydrological observatory of the Wüstebach catchment. Soil hydraulic parameters were derived using inverse modelling with the Hydrus-1D model using the global optimization scheme SCE-UA and soil moisture data from a wireless soil moisture sensor network. The estimated parameters were then used for 3D simulations of water transport using the integrated parallel simulation platform ParFlow-CLM. The simulated soil moisture dynamics, as well as evapotranspiration (ET) and runoff, were compared with long-term field observations to illustrate how well the model was able to reproduce the water budget dynamics. We investigated different anisotropies of hydraulic conductivity to analyze how fast lateral flow processes above the underlying bedrock affect the simulation results. For a detail investigation of the model results we applied the empirical orthogonal function (EOF) and wavelet coherence methods. The EOF analysis of temporal–spatial patterns of simulated and observed soil moisture revealed that introduction of heterogeneity in the soil porosity effectively improves estimates of soil moisture patterns. Our wavelet coherence analysis indicates that wet and dry seasons have significant effect on temporal correlation between observed and simulated soil moisture and ET. Our study demonstrates the usefulness of the EOF and wavelet coherence methods for a more in-depth validation of spatially highly resolved hydrological 3D models.
    Keywords: 3d Hydrological Simulation ; Soil Moisture ; Eof Analysis ; Wavelet Coherence Analysis ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 2
    Language: English
    In: Journal of Hydrology, 2011, Vol.399(3), pp.410-421
    Description: ► Top soil moisture observations for estimation of hydraulic parameters. ► Simultaneous update of model states (soil moisture) and hydraulic parameters. ► SIR-PF for propagation of non-Gaussian distributions through a nonlinear model. ► Estimation of hydr. parameters driven by non linearity between SM and pressure head. In a synthetic study we explore the potential of using surface soil moisture measurements obtained from different satellite platforms to retrieve soil moisture profiles and soil hydraulic properties using a sequential data assimilation procedure and a 1D mechanistic soil water model. Four different homogeneous soil types were investigated including loamy sand, loam, silt, and clayey soils. The forcing data including precipitation and potential evapotranspiration were taken from the meteorological station of Aachen (Germany). With the aid of the forward model run, a synthetic data set was designed and observations were generated. The virtual top soil moisture observations were then assimilated to update the states and hydraulic parameters of the model by means of a particle filtering data assimilation method. Our analyses include the effect of assimilation strategy, measurement frequency, accuracy in surface soil moisture measurements, and soils differing in textural and hydraulic properties. With this approach we were able to assess the value of periodic spaceborne observations of top soil moisture for soil moisture profile estimation and identify the adequate conditions (e.g. temporal resolution and measurement accuracy) for remotely sensed soil moisture data assimilation. Updating of both hydraulic parameters and state variables allowed better predictions of top soil moisture contents as compared with updating of states only. An important conclusion is that the assimilation of remotely-sensed top soil moisture for soil hydraulic parameter estimation generates a bias depending on the soil type. Results indicate that the ability of a data assimilation system to correct the soil moisture state and estimate hydraulic parameters is driven by the non linearity between soil moisture and pressure head.
    Keywords: Soil Moisture ; Data Assimilation ; Particle Filter ; Sequential Importance Resampling ; Hydrus-1d ; Smos ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Journal of Hydrology, May 2016, Vol.536, pp.365-375
    Description: In distributed hydrological modelling one often faces the problem that input data need to be aggregated to match the model resolution. However, aggregated data may be too coarse for the parametrization of the processes represented. This dilemma can be circumvented by the adjustment of certain model parameters. For instance, the reduction of local hydraulic gradients due to spatial aggregation can be partially compensated by increasing soil hydraulic conductivity. In this study, we employed the information entropy concept for the scale dependent parameterization of soil hydraulic conductivity. The loss of information content of terrain curvature as consequence of spatial aggregation was used to determine an amplification factor for soil hydraulic conductivity to compensate the resulting retardation of water flow. To test the usefulness of this approach, continuous 3D hydrological simulations were conducted with different spatial resolutions in the highly instrumented Wüstebach catchment, Germany. Our results indicated that the introduction of an amplification factor can effectively improve model performances both in terms of soil moisture and runoff simulation. However, comparing simulated soil moisture pattern with observation indicated that uniform application of an amplification factor can lead to local overcorrection of soil hydraulic conductivity. This problem could be circumvented by applying the amplification factor only to model grid cells that suffer from high information loss. To this end, we tested two schemes to define appropriate location-specific correction factors. Both schemes led to improved model performance both in terms of soil water content and runoff simulation. Thus, we anticipate that our proposed scaling approach is useful for the application of next-generation hyper-resolution global land surface models.
    Keywords: Scale Dependent Parameterization ; 3d Hydrological Modelling ; Topographical Information Content ; Soil Hydraulic Conductivity ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 4
    Language: English
    In: Journal of Hydrology, December 2017, Vol.555, pp.31-40
    Description: Forest canopy interception alters the isotopic tracer signal of precipitation leading to significant isotopic differences between open precipitation (δOP) and throughfall (δTF). This has important consequences for the tracer-based modeling of streamwater transit times. Some studies have suggested using a simple static correction to δOP by uniformly increasing it because δTF is rarely available for hydrological modeling. Here, we used data from a 38.5 ha spruce forested headwater catchment where three years of δOP and δTF were available to develop a data driven method that accounts for canopy effects on δOP. Changes in isotopic composition, defined as the difference δTF-δOP, varied seasonally with higher values during winter and lower values during summer. We used this pattern to derive a corrected δOP time series and analyzed the impact of using (1) δOP, (2) reference throughfall data (δTF ) and (3) the corrected δOP time series (δOP ) in estimating the fraction of young water ( ), i.e., the percentage of streamflow younger than two to three months. We found that derived from δOP came closer to δTF in comparison to δOP. Thus, a seasonally-varying correction for δOP can be successfully used to infer δTF where it is not available and is superior to the method of using a fixed correction factor. Seasonal isotopic enrichment patterns should be accounted for when estimating and more generally in catchment hydrology studies using other tracer methods to reduce uncertainty.
    Keywords: Isotope Hydrology ; Throughfall ; Fraction of Young Water ; Catchment Hydrology ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
    Source: ScienceDirect Journals (Elsevier)
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  • 5
    Language: English
    In: Journal of Hydrology, May 2015, Vol.524, pp.680-695
    Description: Spatially highly resolved mapping of aquifer heterogeneities is critical for the accurate prediction of groundwater flow and contaminant transport. Here, we demonstrate the value of using full-waveform inversion of crosshole ground penetrating radar (GPR) data for aquifer characterization. We analyze field data from the Krauthausen test site, where crosshole GPR data were acquired along a transect of 20 m length and 10 m depth. Densely spaced cone penetration tests (CPT), located close to the GPR transect, were used to validate and interpret the tomographic images obtained from GPR. A strong correlation was observed between CPT porosity logs and porosity estimates derived from GPR using the Complex Refractive Index Model (CRIM). A less pronounced correlation was observed between electrical conductivity data derived from GPR and CPT. Cluster analysis of the GPR data defined three different subsurface facies, which were found to correspond to sediments with different grain size and porosity. In conclusion, our study suggests that full-waveform inversion of crosshole GPR data followed by cluster analysis is an applicable approach to identify hydrogeological facies in alluvial aquifers and to map their architecture and connectivity. Such facies maps provide valuable information about the subsurface heterogeneity and can be used to construct geologically realistic subsurface models for numerical flow and transport prediction.
    Keywords: Heterogeneity ; Aquifer Characterization ; Geophysical Methods ; Ground Penetrating Radar ; Full-Waveform Inversion ; Cone Penetration Tests ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 6
    Language: English
    In: Journal of Hydrology, October 2016, Vol.541, pp.952-964
    Description: The streamwater transit time distribution (TTD) of a catchment is used to derive insights into the movement of precipitation water via various flow paths to the catchment’s stream. Typically, TTDs are estimated by using the convolution integral to model a weekly tracer signal measured in streamflow. Another approach for evaluating the transit time of water to the catchment stream is the fraction of young water ( ) in streamflow that is younger than a certain threshold age, which also relies on tracer data. However, few studies used tracer data with a higher sampling frequency than weekly. To investigate the influence of the sampling frequency of tracer data on estimates of TTD and , we estimated both indicators for a humid, mesoscale catchment in Germany using tracer data of weekly and higher sampling frequency. We made use of a 1.5 year long time series of daily to sub-daily precipitation and streamwater isotope measurements, which were aggregated to create the weekly resolution data set. We found that a higher sampling frequency improved the stream isotope simulation compared to a weekly one (0.35 vs. 0.24 Nash-Sutcliffe Efficiency) and showed more pronounced short-term dynamics in the simulation result. The TTD based on the high temporal resolution data was considerably different from the weekly one with a shift towards faster transit times, while its corresponding mean transit time of water particles was approximately reduced by half (from 9.5 to 5 years). Similar to this, almost doubled when applying high resolution data compared to weekly one. Thus, the different approaches yield similar results and strongly support each other. This indicates that weekly isotope tracer data lack information about faster water transport mechanisms in the catchment. Thus, we conclude that a higher than weekly sampling frequency should be preferred when investigating a catchment’s water transport characteristics. When comparing TTDs or of different catchments, the temporal resolution of the used datasets needs to be considered.
    Keywords: Catchment Hydrology ; Sampling Frequency ; Transit Time Distribution ; Sine Wave Fitting ; Young Water Fraction ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 April 2016, Vol.178, pp.76-86
    Description: Hydroxylamine (NH OH), a reactive intermediate of several microbial nitrogen turnover processes, is a potential precursor of nitrous oxide (N O) formation in the soil. However, the contribution of soil NH OH to soil N O emission rates in natural ecosystems is unclear. Here, we determined the spatial variability of NH OH content and potential N O emission rates of organic (Oh) and mineral (Ah) soil layers of a Norway spruce forest, using a recently developed analytical method for the determination of soil NH OH content, combined with a geostatistical Kriging approach. Potential soil N O emission rates were determined by laboratory incubations under oxic conditions, followed by gas chromatographic analysis and complemented by ancillary measurements of soil characteristics. Stepwise multiple regressions demonstrated that the potential N O emission rates, NH OH and nitrate (NO ) content were spatially highly correlated, with hotspots for all three parameters observed in the headwater of a small creek flowing through the sampling area. In contrast, soil ammonium (NH ) was only weakly correlated with potential N O emission rates, and was excluded from the multiple regression models. While soil NH OH content explained the potential soil N O emission rates best for both layers, also NO and Mn content turned out to be significant parameters explaining N O formation in both soil layers. The Kriging approach was improved markedly by the addition of the co-variable information of soil NH OH and NO content. The results indicate that determination of soil NH OH content could provide crucial information for the prediction of the spatial variability of soil N O emissions.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Geoderma, 01 August 2017, Vol.299, pp.43-53
    Description: The release and stability of soil water-dispersible colloids (WDC) in the soil structure are critical for colloid-facilitated soil organic carbon sequestration and contaminants transport. In this study, the potential effects of temperature and associated organic carbon (OC) on the release of WDCs in three silt loam topsoils with the same clay content (~ 20%) under different land uses were investigated. A soil fractionation method was used for simulating the release of colloids from the soil under environmental conditions where mobilization and sedimentation processes occur sequentially. The surface loading of OC has been characterized by the analysis of organic carbon content of WDC with the measurements of the specific surface area (SSA). The effects of fractionation temperature on colloidal properties (e.g., particle size and zeta potential) were systematically investigated and the aggregation kinetics of WDC in salt electrolyte influenced by temperature was assessed by dynamic light scattering (DLS). Experimental results demonstrated that the amount of extracted WDC from three soils decreased when the fractionation temperature increased. A more rapid sedimentation of WDC at higher temperatures outweighed the effect of temperature on WDC mobilization from bulk soil in the shaking step. The sedimentation of WDC at various temperatures indicated that the temperature dependence of the water viscosity ( ) was a dominate parameter and caused lower efficiency of WDC mass gained at higher temperature according to the Stoke's law. After introducing the factor of / , the temperature effect only on WDC mobilization during shaking step could be described and the whole fractionation process could be successfully timely determined along the two shaking and sedimentation steps. Activation energies ( ) of about 10 kJ mol could be now calculated for the WDC mobilization processes from the three topsoils. The associated organic carbon contents of WDC (WDC(OC)) and the mineral surface of WDC blocked by organic carbon ( ) after various shaking temperatures and shaking time were further determined in order to examine the WDC(OC) effect on the release of WDC from soil matrix. The results demonstrated that the escape of the mobile clay fraction ( ) from soil at short shaking times is favored by the presence of effective surface loading by an OC layer ( ), which is known to stabilize its colloidal state through electrosteric effects. The WDC(OC) surface concentration has been also used to estimate the clay-associated OC distribution in the three topsoils. In Ca solution, an increase of temperature favors the colloidal stability of WDC as measured from the shift of critical coagulation concentration ( ) to higher concentrations of Ca . In total, the results from this study revealed that temperature and WDC(OC) distribution are critical parameters when considering soil WDC release and stability in natural bulk soils.
    Keywords: Mobilization of Water-Dispersible Colloid ; Temperature ; Soil Fractionation ; Organic Carbon ; Specific Surface Area ; Aggregation Kinetics ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 9
    Language: English
    In: Geoderma, February 2013, Vol.193-194, pp.94-101
    Description: The effects of soil depth on the colloidal properties and potential release of water-dispersible colloids (WDCs) in an agricultural soil were investigated in batch experiments. Nine soil samples were collected along a trench at different depths (from − 7.5 cm to − 127.5 cm) from the TERENO test site located in Selhausen (Germany). A physical gentle soil fractionation confirmed the importance of the solution chemistry in controlling the depth distribution of WDC in the Luvisol soil. A maximal fraction of total clay (f ) equal to approx. 45% was released at a depth of − 37.5 cm to − 52.5 cm below the surface. This was found to be related to the lowest electrical conductivity of the soil electrolyte phase where Ca concentration played a major role. A rather constant content of metal oxyhydroxide (MO(OH)) accounting for about 10% in WDCs was found along the soil depth after a dithionite-citrate-bicarbonate (DCB) extraction. However, a decrease from 40% to 25% in the contribution of MO(OH) particles to SSA of WDC was calculated along the soil depth. Using photon correlation spectroscopy (PCS) and N gas adsorption method, variations were measured in colloidal hydrodynamic diameters (d ) and the mineral specific surface area (SSA) for WDC in water-dispersed and dried states, respectively. A function structure f(n ) was defined to relate the particle size variations between the two states along the soil depth. An inverse correlation was established between the particle size variations which assumed that the number of primary particle in the water-dispersed state increases in subsoil samples. The effects of different soil parameters such as pH, organic carbon (OC) coverage at MO(OH) surfaces and humification index (HIX) of dissolved organic carbon (DOC) on the WDC structure variations are discussed. It can be hypothesized that a maximal surface OC coverage at MO(OH) surfaces of WDCs facilitates the release of small particles through electrosteric dispersion in topsoil. Lower pHs and WDC(OC) content would favor the release of large WDC particles in subsoil where an OC content of a more aliphatic character can also facilitate hydrophobic interactions. ► Solution chemistry controls the depth distribution of WDC in a Luvisol soil. ► SSA of mineral primary particles in WDC from N gas method and DCB extraction ► Contribution of metal oxyhydroxide particles to WDC surface area is depth-dependent. ► Colloidal/primary particle size ratio of WDC is depth-dependent. ► Surface OC coverage and pH control WDC mechanism release.
    Keywords: Water-Dispersible Colloid ; Soil Organic Carbon ; Metal Oxyhydroxide ; Specific Surface Area ; Dissolved Organic Carbon ; Soil Depth Profile ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 10
    Language: English
    In: Vadose Zone Journal, 2016, Vol.15(7), p.0
    Description: A dynamic tension-controlled bottom boundary of lysimeters allows observing water and matter fluxes in lysimeters that are close to natural field conditions, as pressure heads at the lysimeter bottom are adjusted to measured pressure heads at the same depth in the surrounding field. However lysimeters are often transferred from their sampling location for practical reasons or to study, for example, the effect of climate change on soil functions. This transfer can be accompanied by a change aboveground but also in subsurface conditions that are used to control the bottom boundary and that may affect the soil water balance of lysimeters. This issue is also relevant for lysimeter stations which use a tension-controlled bottom boundary and are not directly installed near the site of excavation. The potential impact of different bottom boundary conditions on the water balance of lysimeters that were transferred in a climate impact experiment (SOILCan) was investigated exemplarily by a numerical study. Results showed that by using nonappropriate pressure heads, which were measured in soil profiles with a different texture and water table depth than the profile where the lysimeter was taken from, had partially large impacts on soil water fluxes, especially when the water table was located within a specific critical range. Different climate conditions between sampling and installation site were buffered by the soil and did not show a strong influence on the bottom boundary control of lysimeters when the groundwater table depth was assumed to remain constant. Considering a change in groundwater table depths due to changing climate tempered the effects of climate change on the soil water balance terms. In general, results demonstrate the importance of a proper control of the lysimeters bottom boundary conditions in studies that investigate the influence of climate change on soil functions and ecosystem variables by transferring lysimeter along climate gradients.
    Keywords: Water Balance ; Climate Change ; Soils ; Water Table ; Climate Change ; Climates ; Pressure Head ; Water Depth ; Boundaries ; Lysimeters ; Sampling ; Soil Water ; Groundwater ; Methods and Instruments ; General ; Bl, Bad Lauchstaedt ; Dd, Dedelow ; Eta, Evapotranspiration ; Etp, Potential Evapotranspiration ; Lai, Leaf Area Index ; Sb, Sauerbach ; Se, Selhausen;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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