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  • 1
    Language: English
    In: Vadose Zone Journal, 2013, Vol.12(3), p.0
    Description: Soils are structured on multiple spatial scales, originating from inhomogeneities of the parent material, pedogenesis, soil organisms, plant roots, or tillage. This leads to heterogeneities that cause variability of local measurements of hydraulic state variables and affects the flow behavior of water in soil. Whereas in real-world systems, the true underlying structures can never be absolutely known, it is appealing to employ synthetic or "virtual" experiments for assessing general properties of flow in porous media and grasping the main physical mechanisms. With this aim, three two-dimensional virtual realities with increasing structural complexity, representing cultivated soils with hierarchical spatial heterogeneity on multiple scales were constructed by the interdisciplinary research group Virtual Institute of the Helmholtz Association (INVEST). At these systems, numerical simulations of water dynamics including a heavy rain, a redistribution, and a long-lasting evaporation period were performed. The technical aspects of the construction of the virtual soils and results of the forward simulations have been presented in a paper by Schluter et al. (2012). In this follow-up paper, we use inverse modeling to investigate measurements in virtual vertical soil profiles, mimicking typical field monitoring campaigns with moisture content and matric potential sensors placed at five depths. Contrary to the real situation, we can interpret observed data, their variability, estimated hydraulic properties, and predicted water balance in the light of the known truth. Our results showed that measurements, particularly those of water contents, varied strongly with measuring position. Using data from single profiles in systems similar to our virtual soils thus will lead to very different estimates of the soil hydraulic properties. As a consequence, the correct calculation of the water balance is rather a lucky coincidence than the rule. However, the average of the predicted water balances obtained from the one-dimensional simulations, and the estimated soil hydraulic properties agreed very well with those attained from the two-dimensional systems.
    Keywords: Soils ; Hydrogeology ; Boundary Interactions ; Evaporation ; Grain Size ; Heterogeneous Materials ; Hydrodynamics ; Infiltration ; Interpretation ; Inverse Problem ; Irrigation ; Matric Head ; Measurement ; Moisture ; One-Dimensional Models ; Quantitative Analysis ; Simulation ; Size Distribution ; Soils ; Spatial Distribution ; Tdr Data ; Two-Dimensional Models ; Unsaturated Zone ; Van Genuchten-Mualem Parameters ; Water ; Water Balance;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 2
    Language: English
    In: Vadose Zone Journal, 2012, Vol.11(4), p.0
    Description: The hydraulic behavior of soil is determined by the spatial heterogeneity of its hydraulic properties. The interplay among parent material, pedogenesis, and tillage leads to characteristic structures in cultivated soils. Tillage-induced features like a loosely aggregated seed bed, a compacted plow pan, and soil compaction beneath tractor ruts overlay natural features such as facies and horizons. Assessing the impact of such structural components on vadose zone hydrology requires an observation scale of several meters and a resolution in the range of centimeters, which is not feasible with experimental setups. An alternative solution is the generation of synthetic but realistic structures and their hydraulic properties as a basis for modeling the hydraulic behavior in response to different boundary conditions. With such "virtual soils" at hand, comparative studies are possible that help explore the relation between soil architecture and soil function. We developed a structure generator that provides great flexibility in the design of virtual soils with nested heterogeneity. Virtual soils with increasing complexity were generated to explore scenarios of precipitation and evaporation for a period of several months. The simulations demonstrated that the structure and the hydraulic properties close to the soil surface originating from tillage clearly govern atmospheric boundary fluxes, while the impact of heterogeneity on groundwater recharge is more complex due to threshold effects, hydraulic nonequilibrium, and the interaction with atmospheric forcing. A comparison with one-dimensional, effective representations of these virtual soils demonstrated that upscaling of soil water dynamics becomes inaccurate when lateral fluxes become relevant at the scale of observation.
    Keywords: Hydrogeology ; Soils ; Agriculture ; Air ; Aquifers ; Boundary Conditions ; Ground Water ; Heterogeneity ; Hydraulic Conductivity ; Hydrodynamics ; Moisture ; Morphology ; Recharge ; Simulation ; Soil-Atmosphere Interface ; Soils ; Tillage ; Topsoil ; Unsaturated Zone ; Virtual Reality ; Water;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 3
    Language: English
    In: Vadose Zone Journal, 2011, Vol.10(3), p.988
    Description: Recent studies have shown that rhizosphere hydraulic properties may differ from those of the bulk soil. Specifically, mucilage at the root-soil interface may increase the rhizosphere water holding capacity and hydraulic conductivity during drying. The goal of this study was to point out the implications of such altered rhizosphere hydraulic properties for soil-plant water relations. We addressed this problem through modeling based on a steady-rate approach. We calculated the water flow toward a single root assuming that the rhizosphere and bulk soil were two concentric cylinders having different hydraulic properties. Based on our previous experimental results, we assumed that the rhizosphere had higher water holding capacity and unsaturated conductivity than the bulk soil. The results showed that the water potential gradients in the rhizosphere were much smaller than in the bulk soil. The consequence is that the rhizosphere attenuated and delayed the drop in water potential in the vicinity of the root surface when the soil dried. This led to increased water availability to plants, as well as to higher effective conductivity under unsaturated conditions. The reasons were two: (i) thanks to the high unsaturated conductivity of the rhizosphere, the radius of water uptake was extended from the root to the rhizosphere surface; and (ii) thanks to the high soil water capacity of the rhizosphere, the water depletion in the bulk soil was compensated by water depletion in the rhizosphere. We conclude that under the assumed conditions, the rhizosphere works as an optimal hydraulic conductor and as a reservoir of water that can be taken up when water in the bulk soil becomes limiting.
    Keywords: Agriculture;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 4
    Language: English
    In: Vadose Zone Journal, 2009, Vol.8(3), p.805
    Description: It has been speculated that during periods of water deficit, roots may shrink and lose contact with the soil, with a consequent reduction in root water uptake. Due to the opaque nature of soil, however, this process has never been observed in situ for living plants. Through x-ray tomography and image analysis, we have demonstrated the formation and dynamics of air gaps around roots. The high spatial resolution required to image the soil–root gaps was achieved by combining tomography of the entire sample (field of view of 16 by 16 cm, pixel side 0.32 mm) with local tomography of the soil region around the roots (field of view of 5 by 5 cm, pixel side 0.09 mm). For a sandy soil, we found that when the soil dries to a water content of 0.025 m3 m–3, gaps occur around the taproot and the lateral roots of lupin (Lupinus albus L.). Gaps were larger for the taproot than the laterals and were caused primarily by root shrinkage rather than by soil shrinkage. When the soil was irrigated again, the roots swelled, partially refilling the gaps; however, large gaps persisted in the more proximal, older part of the taproot. Gaps are expected to reduce water transfers between soil and roots. Opening and closing of gaps may help plants to prevent water loss when the soil dries, and to restore the soil–root continuity when water becomes available. The persistence of gaps in the more proximal parts is one reason why roots preferentially take up water from their more distal parts. ; Includes references ; p. 805-809.
    Keywords: Soil Water Content ; Roots ; Soil-Plant Interactions ; Shrinkage ; Plants ; Translocation (Plant Physiology) ; Lupinus Albus ; Forage Legumes ; Spatial Variation ; Drought ; Water Stress ; Sandy Soils ; Water Uptake ; Computed Tomography ; Forage Crops ; Image Analysis ; Taproots;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
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  • 5
    Language: English
    In: Geoderma, 15 July 2019, Vol.346, pp.52-62
    Description: Some soil physical properties can easily be measured using classical laboratory methods. However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This requires non-destructive measurements such as X-ray computed tomography (CT). However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This paper combines parameters obtained from CT analysis (mean macropore diameter, macroporosity, pore connectivity, anisotropy) and classical laboratory methods (dry bulk and aggregate density, saturated hydraulic conductivity, mechanical precompression stress) to analyse soil compaction, exemplified on samples from two tillage treatments (cultivator and plough) and at two moisture states (6 and 1000 kPa matric potential) on a Chernozem collected at a soil depth of 16–22 cm (texture 0–30 cm: silty clay loam). The study shows that the matric potential can have a decisive impact on the mechanical stability of soil. In the loose but less stable plough treatment a more negative matric potential was clearly beneficial to the mechanical stability. In already dense soil structures, as in the cultivator treatment, a reduction of water content was less effective in increasing soil stability. The CT parameters were all closely and uniquely related to each other. The shown CT parameters can be used for a standardized characterization of the soil. Ploughing has a positive effect on soil structure which persists only as long as macroporosity and mean macropore diameter remain high. Plough maintains higher pore connectivity when compacted under dry conditions.
    Keywords: X-Ray CT ; Mechanical Soil Analysis ; Conservation Tillage ; Conventional Tillage ; Soil Compaction ; Precompression Stress ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 6
    In: Water Resources Research, May 2006, Vol.42(5), pp.n/a-n/a
    Description: This paper presents a vision that advocates hydropedology as an advantageous integration of pedology and hydrology for studying the intimate relationships between soil, landscape, and hydrology. Landscape water flux is suggested as a unifying precept for hydropedology, through which pedologic and hydrologic expertise can be better integrated. Landscape water flux here encompasses the source, storage, flux, pathway, residence time, availability, and spatiotemporal distribution of water in the root and deep vadose zones within the landscape. After illustrating multiple knowledge gaps that can be addressed by the synergistic integration of pedology and hydrology, we suggest five scientific hypotheses that are critical to advancing hydropedology and enhancing the prediction of landscape water flux. We then present interlinked strategies for achieving the stated vision. It is our hope that by working together, hydrologists and pedologists, along with scientists in related disciplines, can better guide data acquisition, knowledge integration, and model‐based prediction so as to advance the hydrologic sciences in the next decade and beyond.
    Keywords: Catchment Hydrology ; Landscape Processes ; Scale ; Soil Hydrology ; Soil Physics ; Vadose Zone
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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