Silvicultural operations increasingly aim to achieve desired water-related ecological services of forests and forest soils. Therefore, the effects of forest stand density, site, and soil depth on the water flow types were studied by conducting dye tracer experiments in two montane beech (Fagus sylvatica L.) forests located in the Western Carpathian Volcanic Range, Central Slovakia. Prevalent flow types were identified under usual weather conditions. Brilliant Blue FCF dye tracer was periodically applied in powder form on 1-m(2) plots in forest stands with natural and reduced stand densities, as well as in clear-cuts. When cumulative precipitation reached approximately 100 mm, soil pits were excavated. The dye patterns on the exposed profiles were photographed, and dye coverage, relative dye concentration, and stain widths were determined at various soil depths using image analysis. These patterns were used to discriminate two flow types. Continuous vertical dye plumes were interpreted as an indicator of surface-controlled flow type, which includes heterogeneous infiltration and macropore flow. The matrix-controlled flow type includes both homogeneous and heterogeneous matrix flow, as well as fingering. The log-linear analysis revealed that forest stand density and soil depth were significantly related to the soil water flow type. Preferential flow resulted from heterogeneous infiltration and fingering in the clear-cuts, from heterogeneous infiltration in the natural stands, and from macropore flow in the shelterwood stands. Distinct humus forms and skeleton fraction played a crucial role for various flow patterns observed in these beech stands.

Time domain reflectometry (TDR) trace analysis aims at extracting the water content profile, along TDR probes. This can be done by applying a TDR forward solver inversely. Thus, TDR-trace inversion is basically an optimization problem. As in any optimization procedure, it is worthwhile to include as much a priori information as possible about the problem to be solved. In this study, we discuss the feasibility to use the apparent electrical conductivity as constraint for the TDR inversion. The resistors-in-parallel circuit can be used to integrate a multislice soil model to obtain the apparent electrical conductivity. We apply additionally Archie's law to link the water content of a particular slice with its electrical conductivity. We compare the results from this approach with measured TDR traces and show that the problem is solved exactly. Finally, we address the thin-layer issue because thin layers with a high permittivity contrast result in a delay of the run time of an electromagnetic pulse. We test numerically whether a similar behavior can be observed for a thin layered electrical conductivity profile. Our results show that the thickness of the soil layer with respect to electrical conductivity has no effect on the apparent electrical conductivity. We conclude that the apparent electrical conductivity is appropriate as boundary condition in TDR inversion as long as a procedure is known to convert the water content of a slice to its electrical conductivity

Aggregation of soil particles is crucial for water flow in the vadose zone. Recent studies demonstrated that the unsaturated water flow through an aggregate pair is controlled by the contacts between aggregates. In these studies, the hydraulic conductivity of an aggregate pair was calculated as the harmonic mean of the conductivities of one aggregate and one contact, and it was fitted increasing the tortuosity from 0.5 to 5. In the present study, we investigated whether the contacts between aggregates control the water flow also in large aggregate packings. Our hypothesis was that unsaturated water flow in aggregate packings is primarily a flow in series through aggregates and contacts, and therefore the hydraulic conductivity of the system is properly approximated with that of an aggregate pair. To verify this hypothesis, we used neutron radiography to image water infiltration and redistribution in a cylindrical packing of aggregates. Then we numerically simulated the experiment using the hydraulic properties obtained from the model of an aggregate pair—i.e., tortuosity equal to 5. The observed water flow showed that the water redistribution following infiltration was particularly slow and the wetting front was very sharp. These features are explained by the bottleneck effect of the contacts, which were rapidly drained and limited the flow. This flow behavior was well reproduced in the simulations with tortuosity equal to 5, while simulations with tortuosity equal to 0.5, which is the tortuosity obtained for the aggregates, could not reproduce the observations. This study shows that the contacts control the unsaturated water flow also in larger aggregate packings and the process can be in a first approximation modeled as a flow in series through aggregates and contacts.

Unsaturated water flow through soil aggregates is controlled by the contacts between aggregates. The contacts are highly conductive when wet and become bottle-necks for flow when drained. We postulate that the hydraulic conductivity of the contacts is in first place determined by the water-filled contact area. The objective of this study was to measure and model the water-filled contact area and to relate it to the conductivity of a series of aggregates. We performed microscopic tomography of an aggregate pair equilibrated at different water potentials. By means of image analysis and a morphological pore network model, the water-filled contact area was calculated. We found that the aggregate surface is rough and the contact region contains macropores which are rapidly drained. As a consequence the water-filled contact area dramatically decreases as the water potential is diminished. We modeled this process by describing the aggregates as spheres covered by much smaller spheres representing the roughness. The water-filled contact was analytically calculated from this model. Knowing the water-filled contact area we up-scale the hydraulic conductivity of a series of aggregates. This is calculated as the harmonic mean of the contact and aggregate conductivities. The contact conductivity is calculated from the water-filled contact area. Near saturation the conductivity of a series of aggregates is close to the conductivity of a single aggregate, and, when further drained, it rapidly decreases as the water-filled contact area. The model matches the experimental data well.

A well-controlled 3-D experiment with pre-defined block heterogeneities is conducted, where neutron tomography is used to map 3-D water distribution after two successive drainage steps. The material and hydraulic properties of the two sands are first measured in the laboratory with multistep outflow experiments. Additionally, the pore structure of the sands is acquired by means of image analysis of synchrotron tomography data and the structure is used for pore-scale simulation of one- and two-phase flow with Lattice-Boltzmann methods. This gives us another set of material and hydraulic parameters of the sands. The two sets of hydraulic properties (from the lab scale and from the pore scale) are then used in numerical simulations of the 3-D experiment.The paper discusses critical aspects and benchmarks for experimental measurements of 3-D water distribution in heterogeneous porous media. Additionally, we discuss possibilities as well as difficulties and limitations in the determination of hydraulic properties of materials using two conceptually different approaches (pore scale and lab scale). We then test with the numerical simulations how good can predictions on flow and water content in structured media be when using these state-of-the-art methods for the determination of hydraulic properties. Based on the numerical simulations, we discuss which parameters are more difficult to predict and which of the two approaches (lab scale or pore scale) enables better predictions. (C) 2007 Elsevier Ltd. All rights reserved.

The crown vegetation of a deciduous forest is known to be semitransparent at low microwave frequencies, and leaf litter covering the forest soil has been recognized to have a significant impact on ground emission. The proposed approach for modeling the L-band radiative transfer through leaf litter consists of an isotropic effective medium approach for the litter permittivities, a coherent radiative transfer model for computing the coherent reflectivities from dielectric depth profiles, and an averaging procedure for computing the reflectivities determining the field-scale brightness temperatures. Evaluations were performed for the case of leaf litter on top of a conducting wire grid (litter-grid formation) and for litter on underlying soil (litter-soil formation). A model sensitivity analysis was performed with respect to parameters characterizing litter thickness variations and boundary roughness. For the litter-soil formation, the model was rather sensitive to local irregularities at the air-to-litter boundary. Modeled microwave signatures reproduced the major features of the measurements performed on a site comprising a litter-grid formation. Under dry conditions, the investigated litter layer was nearly ldquoinvisible.rdquo When the same litter layer was wetted, it acted as an important radiation source to be taken into account for the quantitative remote soil moisture detection of forested areas. Under certain conditions, the simulations revealed an increasing brightness when the litter is wetted prior to the underlying soil. Further wetting of the litter-soil system then resulted in a decreasing brightness as expected for increased moisture. Such effects are important to know to avoid misleading interpretations of L-band signatures.

The temporal and spatial distribution of water within a porous medium is affected by the medium’s structure, i.e., the spatial arrangement of its constituents. To analyze structural effects on the fluid dynamics, we measured the 3D water content distribution in a heterogeneous sand column during two drainage-wetting cycles using neutron transmission tomography. The sample with a volume of 105cm3 contained 101 cubes of fine and 49 cubes of coarse sand with particles ranging from 0.01 to 0.05 and 0.03 to 0.09cm, respectively. The pressure at the lower boundary was determined by the water reservoir positioned between 7 and 39cm below the top of the column. The duration of one complete 3D scanning with a spatial resolution of 127μm was 56s. The signal to noise ratio of the measurements was low due to the short exposure time in the neutron beam, but it was possible to quantify the water content in the individual cubes and hence the effect of structure on macroscopic water distribution. Continuous structures of coarse sand drained faster than coarse sand without connection to the upper boundary. During the initial wetting phase, cubes of coarse sand material completely embedded in the fine material remained water unsaturated due to air entrapment. The effect of the coarse sand connectivity was analyzed in two-dimensional numerical simulations based on Richards equation. In contrast to the measurements, no effect of structure connectivity was found. The coarse sand cubes embedded within the fine matrix drain as quickly as the coarse sand cubes arranged in a continuous channel due to the model assumption of a continuous air phase.

The study of mechanisms controlling preferential flow and transport in variably saturated fractured clayey till is often hindered by insufficient spatial resolution or unknown measuring volume. With the objective to study these mechanisms while circumventing the obstacles, tracer experiments with two fluorescent tracers Acid Yellow 7 (AY7) and Sulforhodamine B (SB) were performed at three different rain events for a fall and a summer season. Irrigated areas were excavated down to depths of 2.8 m and the movement of both tracers in the exposed profiles was delineated simultaneously by high spatial resolution apparent concentration maps (pixel approximately 1 mm(2)) obtained with an imaging device. The device consists of a light source and a CCD camera, both equipped with tracer-specific-filters for fluorescent light. The fluorescence images were corrected for nonuniform lighting, changing surface roughness, and varying optical properties of the soil profile. The resulting two-dimensional apparent concentration distribution profiles of the tracers showed that: (i) relative low water content in the upper 10 cm of the irrigated till in summer had a pronounced retardation effect on the AY7-migration and no effect on the SB-migration; (ii) the dead-end biopores were not activated in the fall season; (iii) only 3D fracture-plans connected to hydraulically active 1D-biopores contributed to the leaching; (iv) the tracer migration primary followed macropores during both seasons, though AY7 also followed a topsoil piston transport in summer; (v) the highest tracer pixel apparent concentrations were often found in macropores and most pronounced in the summer season; and (vi) 3D-dilution in fractures seems to play a dominating role in AY7-migration in the fall season.

The microwave Forest Soil Moisture Experiment (FOSMEX) was performed at a deciduous forest site at the Research Centre Julich (Germany). An L- and an X-band radiometer were mounted 100 m above ground and directed to the canopy. The measurements consist of dual- and single-polarized L- and X-band data and simultaneously recorded ground moisture, temperature, and meteorological data. The canopy L-band transmissivity was estimated from a subset of the FOSMEX data, where the ground was masked with a metalized foil. For the foliage-free canopy, the reflecting foil diminished the L-band brightness by ap24 K, whereas brightness increased by ap14 K when the foil was removed from below the foliated canopy. Depending on the assumption made on the scattering albedo of the canopy, the transmissivities were between 0.2 and 0.51. Furthermore, the contribution of the foliage was quantified. Although, the evaluation revealed the semitransparency of the canopy for L-band frequencies, the brightness sensitivity with respect to ground moisture was substantially reduced for all foliation states. The effect of ground surface moisture was explored in an irrigation experiment. The L-band measurements were only affected for a few hours until the water drained through the litter layer. This emphasizes the significance of the presence of litter for soil moisture retrieval from remotely sensed L-band brightness data. The FOSMEX database serves for further testing and improving radiative transfer models used for interpreting microwave data received from future spaceborne L-band radiometers flying over areas comprising a considerable fraction of deciduous forests.

In this study, we discuss the consistence of measured and calculated TDR traces. The calculated traces are solutions of a time domain reflectometry (TDR) forward solver, an algorithm for a computing the TDR trace for a given dielectric profile along a transmission line. An unambiguous and efficient forward solver is a prerequisite for a good solution of the inverse problem, i.e., to extract the spatial distribution of the dielectric properties along the transmission line from a TDR trace. To advance our understanding of TDR inversion, we proceeded in two steps: (1) design of a TDR head section with minimal disturbances on the signal and (2) searching for causes why measured and predicted TDR traces differ. Based on a first experiment with a three-rod TDR probe of 100 cm length, we demonstrated that our TDR forward solver-like others presented in literature-approximate the measured TDR traces apparently well but not precisely enough for signal inversion. In a second experiment, using a two-rod TDR probe of 70 cm length, we addressed the problem of non-parallel transmission lines. We found that the influence of a non-parallel installation is similar to an increase of the electrical conductivity in soil water but can be distinguished from this property. A third experiment reveals that lateral and longitudinal disturbances in the vicinity of a TDR probe are of minor importance. From the analysis of our experiments, we found that neither lateral disturbances nor non-parallel rods are responsible for the deviations between calculated and measured traces. This analysis showed us that structure in the sampled medium affects the shape of the TDR traces. Since minor deviations are essential for TDR-signal inversion, we need new concepts to handle the fuzziness between measurements and calculations.

To predict fluid phase distribution in porous media, the effect of geometric properties on flow processes must be understood. In this study, we analyze the effect of volume, surface, curvature and connectivity (the four Minkowski functionals) on the hydraulic conductivity and the water retention curve. For that purpose, we generated 12 artificial structures with 8003 voxels (the units of a 3D image) and compared them with a scanned sand sample of the same size. The structures were generated with a Boolean model based on a random distribution of overlapping ellipsoids whose size and shape were chosen to fulfill the criteria of the measured functionals. The pore structure of sand material was mapped with X-rays from synchrotrons.

This study aims at modeling the transport of a conservative tracer in two dimensions, as experimentally observed in a strongly heterogeneous medium under conditions of variable water saturation during drainage and imbibition. Solute transport experiments were conducted in a sand tank containing an artificial packing of three quartz sands of different particle sizes. The packing was characterized by the presence of numerous homogeneous layers (0.5 x 5 x 5 cm) inclined at 45 degrees and randomly distributed in a tank. Six different stationary flow conditions were sequentially established during imbibition and drainage. When a stationary flow regime was reached, several solute pulses were applied at different positions at the upper surface of the sand structure. The transport regime was studied by monitoring the tracer plumes injected as point-like pulses at the surface, as they travelled through the sand bedding.A textural map was generated from a digital image of the sand bedding. The Richards equation was solved with the experimental boundary conditions assuming homogeneity of the individual sand layers. The hydraulic properties of the three quartz sands were deduced from multistep-outflow column experiments [Ursino N, Gimmi T. Combined effect of heterogeneity, anisotropy and saturation on steady state flow and transport: structure recognition and numerical simulation. Water Resour Res 2004;40. doi:10.1029/2003WR002180]. The convection-dispersion equation was solved on the resulting flow fields for solute pulses of given solute mass applied onto the top boundary at the same positions as in the experiment. The simulated and observed solute concentration distributions were then compared. In agreement with the experimental observations, the simulations reproduced the existence of preferential pathways in those stationary flow fields at low saturation degrees. The values of the vertical and horizontal macroscopic dispersivities obtained from the simulations are smaller than experimentally observed, especially at low flow rates. The simulated solute concentration distributions show a realistic degree of solute dilution quantified as reactor ratio. (C) 2008 Elsevier Ltd. All rights reserved.

We studied the flow of water and the transport of solutes in a heterogeneously packed sand tank by performing transport experiments with a fluorescent tracer. The packing consisted of an inclined, layered sand bedding characterized by a large number of thin layers of three quartz sands of different grain sizes. The purpose was to evaluate to which extent the preceding wetting‐draining history affects the mixing of solutes in presence of textural heterogeneities. We irrigated the surface at stepwise increased flow rates (imbibition) and afterward at stepwise decreased flow rates (drainage). At each step the flow rate was kept constant to monitor the transport and spreading of several tracer plumes in the stationary flow field. The results show that during imbibition, mixing at the textural boundaries is hindered and the solutes are funneled through connected preferential flow paths. During drainage we observed enhanced mixing resulting in a definitely less heterogeneous transport regime. These results stress the role of the preceding wetting‐draining history for determining the characteristics of solute transport in heterogeneous media. We demonstrate this using the integral mixing characteristics “dilution index” and “reactor ratio,” which quantify the degree of preferentiality of flow and transport.

Water flow between porous grains varies widely depending on the water distribution in contacts between grains. The hydraulic behavior of contacts varies from highly conductive when water fills the contacts to a bottleneck to flow as water pressure drops and contact asperities rapidly drain. Such changes greatly impact the hydraulic conductivity of porous grain packs such as aggregated soil. The dominant driving force of water flow across contacts is capillarity, often quantified relative to gravity and viscous forces using the capillary and Bond numbers. For fast water infiltration, viscous forces dominate. For simplicity we modeled the water distribution between spherical porous grains whose surfaces are covered by spherical bumps of much smaller radii. We provide experimental evidence obtained by neutron radiography and synchrotron-based x-ray tomographic microscopy documenting transitions in the flow behavior across contacts.

Soils are often structured as fine-porous aggregates separated by large inter-aggregate pores. Under unsaturated conditions, water is mostly stored in the aggregates and water flow depends on the properties of the aggregates as well as on those of the contacts between aggregates. The goal of this study is to model and evaluate the hydraulic properties of the contacts. We used neutron radiography to monitor the infiltration of water through series of aggregates. The flow process was numerically simulated by considering the hydraulically conducting contact area between aggregates as a variable that depends on the capillary pressure. This contact area was evaluated by matching the observed and simulated water flow across aggregates. We determined the conductivity of the contacts assuming that it scales with the contact area. We also measured the equivalent conductivity of series of aggregates. We found that during drainage the hydraulically conducting contact area drastically decreases and the conductivity of the contacts becomes much smaller than that of the aggregates. We also found that the equivalent conductivity of the aggregate series decreases as the conductivity of the contacts. We concluded that the contacts control the flow: they are highly conductive when wet, but act as bottle-necks under drained conditions. The abrupt transition between these two limiting cases indicates that the contact region is more rapidly drained than the aggregate interior. Our findings might be extended to describe water transfer to an evaporating boundary, infiltration and storage of water in unsaturated aggregated soils.