Kooperativer Bibliotheksverbund

Berlin Brandenburg

and
and

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
  • English  (52)
  • Wiley Online Library  (52)
Type of Medium
Language
Year
  • 1
    In: Water Resources Research, March 2007, Vol.43(3), pp.n/a-n/a
    Description: Large‐scale models of transient flow processes in the unsaturated zone require, in general, upscaling of the flow problem in order to capture the impact of heterogeneities on a small scale, which cannot be resolved by the model. Effective parameters for the upscaled models are often derived from second‐order stochastic properties of the parameter fields. Such properties are good quantifications for parameter fields, which are multi‐Gaussian. However, the structure of soil does rarely resemble these kinds of fields. The non‐multi‐Gaussian field properties can lead to strong discrepancies between predictions of upscaled models and the averaged real flow process. In particular, the connected paths of parameter ranges of the medium are important features, which are usually not taken into account in stochastic approaches. They are determined here by the Euler number of one‐cut indicator fields. Methods to predict effective parameters are needed that incorporate this type of information. We discuss different simple and fast approaches for estimating the effective parameter for upscaled models of slow transient flow processes in the unsaturated zone, where connected paths of the material may be taken into account. Upscaled models are derived with the assumption of capillary equilibrium. The effective parameters are calculated using effective media approaches. We also discuss the limits of the applicability of these methods.
    Keywords: Richards Equation ; Unsaturated Flow ; Upscaling
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 2
    In: New Phytologist, November 2011, Vol.192(3), pp.653-663
    Description: • Despite the importance of rhizosphere properties for water flow from soil to roots, there is limited quantitative information on the distribution of water in the rhizosphere of plants. • Here, we used neutron tomography to quantify and visualize the water content in the rhizosphere of the plant species chickpea (Cicer arietinum), white lupin (Lupinus albus), and maize (Zea mays) 12 d after planting. • We clearly observed increasing soil water contents (θ) towards the root surface for all three plant species, as opposed to the usual assumption of decreasing water content. This was true for tap roots and lateral roots of both upper and lower parts of the root system. Furthermore, water gradients around the lower part of the roots were smaller and extended further into bulk soil compared with the upper part, where the gradients in water content were steeper. • Incorporating the hydraulic conductivity and water retention parameters of the rhizosphere into our model, we could simulate the gradual changes of θ towards the root surface, in agreement with the observations. The modelling result suggests that roots in their rhizosphere may modify the hydraulic properties of soil in a way that improves uptake under dry conditions.
    Keywords: Extent Of Rhizosphere ; Modelling ; Neutron Tomography ; Rhizosphere Hydraulic Properties ; Root Water Uptake ; Soil Moisture Profile ; Water Distribution
    ISSN: 0028-646X
    E-ISSN: 1469-8137
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    In: Water Resources Research, May 2014, Vol.50(5), pp.4514-4529
    Description: A major difficulty in modeling multiphase flow in porous media is the emergence of trapped phases. Our experiments demonstrate that gas can be trapped in either single‐pores, multipores, or in large connected networks. These large connected clusters can comprise up to eight grain volumes and can contain up to 50% of the whole trapped gas volume. About 85% of the gas volume is trapped by gas clusters. This variety of possible trapped gas clusters of different shape and volume will lead to a better process understanding of bubble‐mediated mass transfer. Since multipore gas bubbles are in contact with the solid surface through ultrathin adsorbed water films the interfacial area between trapped gas clusters and intergranular capillary water is only about 80% of the total gas surface. We could derive a significant (R = 0.98) linear relationship between the gas‐water‐interface and gas saturation. We found no systematic dependency of the front velocity of the invading water phase in the velocity range from 0.1 to 0.6 cm/min corresponding to capillary numbers from 2 × 10 to 10. Our experimental results indicate that the capillary trapping mechanism is controlled by the local pore structure and local connectivity and not by thermodynamics, i.e., by the minimum of the , at least in the considered velocity range. Consistent with this physical picture is our finding that the trapping frequency (= bubble‐size distribution) reflects the pore size distribution for the whole range of pore radii, i.e., the capillary trapping process is determined by statistics and not by thermodynamics. No systematic dependency of trapping efficiency on capillary number Majority of trapped gas bubbles (85%) are multipore trapped Trapping of gas clusters is determined by statistics and not by thermodynamics
    Keywords: Gas Clusters ; Capillary Trapping ; Interfacial Area
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 4
    In: Geoarchaeology, July 2015, Vol.30(4), pp.369-378
    Description: Roman cisterns served as rainwater storage devices for centuries and are densely distributed in parts of northern Jordan. A major earthquake hit the region . A.D. 750 and in a short time many settlements were abandoned. As a consequence, most cisterns were not maintained, and they filled with sediments that today provide a postabandonment depositional record. In two field surveys, we mapped the locations of more than 100 cisterns in the Wadi Al‐Arab basin and selected two for detailed stratigraphic analysis that included C and optically stimulated luminescence dating. Catchment basin area for each cistern was determined by differential GPS. Both cisterns filled with sediments after the great earthquake and consequent abandonment of the region. Calculated sediment volumes are translated to long‐term average sediment export rates of 2.6–6.6 t haa, which are comparable to erosion and sediment yield rates from other studies within the Mediterranean region. Our pilot study suggests that this approach can be applied elsewhere to calculate long‐term sediment export rates on hill slopes containing relict cisterns.
    Keywords: Quaternary Geology ; Sedimentary Petrology ; Arid Environment ; Asia ; Cenozoic ; Chronostratigraphy ; Clay Minerals ; Climate Change ; Climatic Controls ; Dates ; Depositional Environment ; Desertification ; Drainage Basins ; Erodibility ; Erosion ; Erosion Rates ; Holocene ; Human Activity ; Human Ecology ; Hydrology ; Jordan ; Jordan River ; Land Use ; Mediterranean Region ; Middle Ages ; Middle East ; Optically Stimulated Luminescence ; Paleogeography ; Permeability ; Quaternary ; Rainfall ; Reconstruction ; Roman Period ; Sediment Yield ; Sedimentation ; Sheet Silicates ; Silicates ; Soil Erosion ; Stratigraphy ; Terrestrial Environment ; Upper Holocene ; Urban Environment ; Wadi Al-Arab ; Water Resources;
    ISSN: 0883-6353
    E-ISSN: 1520-6548
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 5
    In: Water Resources Research, November 2015, Vol.51(11), pp.9094-9111
    Description: We study the impact of pore structure and surface roughness on capillary trapping of nonwetting gas phase during imbibition with water for capillary numbers between 10 and 5 × 10, within glass beads, natural sands, glass beads monolayers, and 2‐D micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution, and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the microstructure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass beads monolayers and 2‐D micromodels, we could show that bypass trapping controls the trapping process in glass beads monolayers, while snap‐off trapping controls the trapping process in 2‐D micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs. Moreover, for small capillary numbers of 10, we found that the cluster size distribution of trapped gas clusters of all 2‐D and 3‐D porous media can be described by a universal power law behavior predicted from percolation theory. This cannot be expected a priori for 2‐D porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2‐D micromodels with rough surfaces. Surface roughness controls capillary trapping efficiency The transition‐zone model can be applied to 2‐D micromodels with rough surfaces The 2‐D and 3‐D porous media belong all to the same universality class
    Keywords: Surface Roughness ; Precursor Thin‐Film Flow ; Snap‐Off Trapping ; Universal Power Law ; Ordinary Bond Percolation
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 6
    In: Water Resources Research, June 2017, Vol.53(6), pp.4709-4724
    Description: The relaxation dynamics toward a hydrostatic equilibrium after a change in phase saturation in porous media is governed by fluid reconfiguration at the pore scale. Little is known whether a hydrostatic equilibrium in which all interfaces come to rest is ever reached and which microscopic processes govern the time scales of relaxation. Here we apply fast synchrotron‐based X‐ray tomography (X‐ray CT) to measure the slow relaxation dynamics of fluid interfaces in a glass bead pack after fast drainage of the sample. The relaxation of interfaces triggers internal redistribution of fluids, reduces the surface energy stored in the fluid interfaces, and relaxes the contact angle toward the equilibrium value while the fluid topology remains unchanged. The equilibration of capillary pressures occurs in two stages: (i) a quick relaxation within seconds in which most of the pressure drop that built up during drainage is dissipated, a process that is to fast to be captured with fast X‐ray CT, and (ii) a slow relaxation with characteristic time scales of 1–4 h which manifests itself as a spontaneous imbibition process that is well described by the Washburn equation for capillary rise in porous media. The slow relaxation implies that a hydrostatic equilibrium is hardly ever attained in practice when conducting two‐phase experiments in which a flux boundary condition is changed from flow to no‐flow. Implications for experiments with pressure boundary conditions are discussed. What happens to fluids in a porous medium after pumping is stopped? Fast X‐ray tomography shows that even in a sample smaller than a sugar cube fluid interfaces continue to move for hours until an optimal fluid configuration is reached. The pace is limited by slow relaxation of dynamic contact angles. Therefore hydrostatic equilibrium, which is the state at which all fluid interfaces come to rest, is hardly ever attained in practice when conducting two‐phase flow experiments where the flow is stopped in much larger soil or rock samples. Relaxation dynamics through internal redistribution of fluids after fast drainage occurs in two stages A quick dissipation within seconds is followed by slow relaxation within several hours due to relaxation of dynamic contact angles Fluid configurations during relaxation are very different from those during quasi‐static drainage and imbibition
    Keywords: Two‐Phase Flow ; Dynamic Effects ; Hydraulic Nonequilibrium ; Dynamic Contact Angle ; Fluid Configuration ; Fluid Topology
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 7
    In: Water Resources Research, November 2018, Vol.54(11), pp.9033-9044
    Description: Structural hierarchy is a fundamental characteristic of natural porous media. Yet it provokes one of the grand challenges for the modeling of fluid flow and transport since pore‐scale structures and continuum‐scale domains often coincide independent of the observation scale. Common approaches to represent structural hierarchy build, for example, on a multidomain continuum for transport or on the coupling of the Stokes equations with Darcy's law for fluid flow. These approaches, however, are computationally expensive or introduce empirical parameters that are difficult to derive with independent observations. We present an efficient model for fluid flow based on Darcy's law and the law of Hagen‐Poiseuille that is parameterized based on the explicit pore space morphology obtained, for example, by X‐ray μ‐CT and inherently permits the coupling of pore‐scale and continuum‐scale domain. We used the resulting flow field to predict the transport of solutes via particle tracking across the different domains. Compared to experimental breakthrough data from laboratory‐scale columns with hierarchically structured porosity built from solid glass beads and microporous glass pellets, an excellent agreement was achieved without any calibration. Furthermore, we present different test scenarios to compare the flow fields resulting from the Stokes‐Brinkman equations and our approach to comprehensively illustrate its advantages and limitations. In this way, we could show a striking efficiency and accuracy of our approach that qualifies as general alternative for the modeling of fluid flow and transport in hierarchical porous media, for example, fractured rock or karstic aquifers. A model for the simulation of pore‐scale and continuum‐scale flow in hierarchically structured porous media is developed Explicit pore space morphology obtained by image analysis of X‐ray micro‐CT images is used for parameterization Predictions of solute breakthrough obtained by particle tracking perfectly match observations
    Keywords: Darcy'S Law ; Particle Tracking ; Column Experiments ; X‐Ray Μ‐Ct ; Pore Space Morphology ; Image Analysis
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 8
    In: Ecohydrology, September 2018, Vol.11(6), pp.n/a-n/a
    Description: By applying the newly developed flow cell (FC) concept, this study investigated the impact of small‐scale spatial variations (millimetre to centimetre) in organic matter (OM) composition (diffusive reflectance infrared Fourier transform spectroscopy), biological activity (zymography), and wettability (contact angle [CA]) on transport processes (tracer experiments, radiography). Experiments were conducted in five undisturbed soil slices (millimetre apart), consisting of a sandy matrix with an embedded loamy band. In the loamy band increased enzyme activities and OM (10 mm apart) were found compared with the sand matrix, with no interrelations although spatial autocorrelation ranges were up to 7 cm. CAs were increased (0–110°) above the loamy band and were negatively correlated with acid phosphatase. Missing correlations were probably attributed to texture variations between soil slices. A general correlation between CA and C content (bulk) were confirmed. Variability in texture and hydraulic properties led to the formation of heterogeneous flow patterns and probably to heterogeneously distributed interfacial properties. The new FC concept allows process evaluation on the millimetre scale to analyse spatial relations, that is, between small‐scale textural changes on transport processes and biological responses. The concept has been proved as a versatile tool to analyse spatial distribution of biological and interfacial soil properties in conjunction with the analysis of complex micro‐hydraulic processes for undisturbed soil samples. The concept may be improved by additional nondestructive imaging methods, which is especially challenging for the detection of small‐scale textural changes.
    Keywords: Drift Spectroscopy ; Extracellular Enzyme Activity ; Flow Cell ; Soil Water Repellency ; Transport Processes ; Undisturbed Soil ; X‐Ray Radiography
    ISSN: 1936-0584
    E-ISSN: 1936-0592
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 9
    Language: English
    In: Journal of Plant Nutrition and Soil Science, February 2010, Vol.173(1), pp.88-99
    Description: Soil, the “Earth's thin skin” serves as the delicate interface between the biosphere, hydrosphere, atmosphere, and lithosphere. It is a dynamic and hierarchically organized system of various organic and inorganic constituents and organisms, the spatial structure of which defines a large, complex, and heterogeneous interface. Biogeochemical processes at soil interfaces are fundamental for the overall soil development, and they are the primary driving force for key ecosystem functions such as plant productivity and water quality. Ultimately, these processes control the fate and transport of contaminants and nutrients into the vadose zone and as such their biogeochemical cycling. The definite objective in biogeochemical‐interface research is to gain a mechanistic understanding of the architecture of these biogeochemical interfaces in soils and of the complex interplay and interdependencies of the physical, chemical, and biological processes acting at and within these dynamic interfaces in soil. The major challenges are (1) to identify the factors controlling the architecture of biogeochemical interfaces, (2) to link the processes operative at the individual molecular and/or organism scale to the phenomena active at the aggregate scale in a mechanistic way, and (3) to explain the behavior of organic chemicals in soil within a general mechanistic framework. To put this in action, integration of soil physical, chemical, and biological disciplines is mandatory. Indispensably, it requires the adaption and development of characterization and probing techniques adapted from the neighboring fields of molecular biology, analytical and computational chemistry as well as materials and nano‐sciences. To shape this field of fundamental soil research, the German Research Foundation (DFG) has granted the Priority Program “Biogeochemical Interfaces in Soil”, in which 22 individual research projects are involved.
    Keywords: Soil Function ; Soil Architecture ; Spectro‐Microscopy ; Tomography ; Som ; Soil Microbial Ecology ; Organic Chemicals
    ISSN: 1436-8730
    E-ISSN: 1522-2624
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 10
    In: Water Resources Research, April 2009, Vol.45(4), pp.n/a-n/a
    Description: High‐resolution optical bench‐scale experiments were conducted in order to investigate local gas flow pattern and integral flow properties caused by point‐like gas injection into water‐saturated glass beads. The main goal of this study was to test the validity of the continuum approach for two‐fluid flow in macroscopic homogeneous media. Analyzing the steady state experimental gas flow pattern that satisfies the necessary coherence condition by image processing and calibrating the optical gas distribution by the gravimetrical gas saturation, it was found that a pulse‐like function yields the best fit for the lateral gas saturation profile. This strange behavior of a relatively sharp saturation transition is in contradiction to the widely anticipated picture of a smooth Gaussian‐like transition, which is obtained by the continuum approach. This transition is caused by the channelized flow structure, and it turns out that only a narrow range of capillary pressure is realized by the system, whereas the continuum approach assumes that within the representative elementary volume the whole spectrum of capillary pressures can be realized. It was found that the stochastical hypothesis proposed by Selker et al. (2007) that bridges pore scale and continuum scale is supported by the experiments. In order to study channelized gas flow on the pore scale, a variational treatment, which minimizes the free energy of an undulating capillary, was carried out. On the basis of thermodynamical arguments the geometric form of a microcapillary, macrochannel formation and a length‐scale‐dependent transition in gas flow pattern from coherent to incoherent flow are discussed.
    Keywords: Air Sparging ; Continuum Modeling ; Pore‐Scale Modeling ; Gas Flow Pattern ; Instability Analysis ; Image Processing
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. Further information can be found on the KOBV privacy pages