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Preferential flow has been hypothesized as an important factor for chemical leaching from tile‐drained agricultural fields with structured soils originating from glacial till sediments. Previous studies showed that one‐dimensional single‐porosity models (1D‐SPM) failed and that one‐dimensional dual‐permeability models (1D‐DPERM) were limited in explaining both Br leaching and residual Br distribution, although tile water outflow peaks could somehow be reproduced. The objective of this paper was to analyze the tile outflow and leaching patterns using a two‐dimensional (2D)‐DPERM and a standard 2D‐SPM for comparison. Flow and transport were simulated in a 2D vertical cross‐section of 5.9 m length and 2 m depth using previously tested parameters. Simulated drainage rates and Br‐effluent concentrations were made comparable with collector data from a field experiment by weighing results for irrigated and nonirrigated plots according to their area fractions. The 2D‐DPERM simulations for surface application of Br in dissolved form in both domains overestimated the observed initial outflow concentration peaks, in contrast to closer approximation of observations assuming Br application in the soil matrix domain only. The simulated 2D mass transfer rate distribution showed most intensive exchange between domains near the water table and in the topsoil. Results from the 2D‐DPERM analyses suggest that conditions at the soil surface, near the water table, and of the field‐scale mixing are significantly affecting leaching patterns, in addition to local nonequilibrium effects. Here, the description of preferential flow toward tile drain could be strongly improved with the 2D‐DPERM compared with the 2D‐SPM. Further improvements remain challenging with respect to DPERM numerical modeling and field experimentation, with special attention toward soil structure and soil surface conditions.

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Skins of soil aggregates often consist of clayey or clay‐organic coatings which may affect preferential flow in aggregated soils. The objective was to determine hydraulic properties of samples with intact and removed (cut) skins and interior/skin hydraulic conductivity ratios for estimating mass transfer parameters in dual‐permeability models. Soil aggregates from the Csd‐horizon of a clay‐loam glacial till soil (Stagnic Calcaric Regosol) were analyzed. A tension‐imbibition apparatus was used for measuring water uptake of multiple aggregates at boundary matric potential heads of −1 and −5 cm. Sorptivities were used to calculate mean weighted water diffusivities and final water contents to fit wetting retention functions. Water retention and hydraulic conductivity functions for the skin layer were derived from differences in water contents and hydraulic resistances between intact and cut samples. Water absorption rates were generally smaller for intact than for cut aggregates. The water retention function of cut was shifted towards smaller water contents compared with intact samples. Mean water diffusivity of intact was 4.5 times smaller than that of cut samples. The interior/skin ratio in unsaturated hydraulic conductivity was about 12 in the measured matric potential head range. The ratio was up to 70 near water saturation and dropped below unity for soil water potentials smaller −1000 cm of water. Aggregate skins may be regarded as a separate porous domain whose hydraulic properties may control water transfer between inter‐ and intraaggregate pore domains in structured soils.

[1] Physically based models are increasingly applied to analyze contaminant transport in soil by preferential water flow. Unfortunately, in the past, preferential flow models were rarely evaluated using appropriate experimental data because of the complexity of conceptual models and limitations of measuring techniques. In this study, we designed a novel soil column experiment with advanced measurement techniques that enabled us to discriminate macropore and matrix water flow and quantify interdomain (macroporematrix) water transfer. Experiments of drainage and upward and downward infiltration revealed hydraulic nonequilibrium between matrix and macropore domains. Cumulative interdomain water transfer could be estimated using mass balance calculations. In a hierarchical modeling approach, four numerical models of different complexity were compared to the column experiment data. As a reference model, pseudo three-dimensional axisymmetric Richards' equation (ARE) was used for inverse estimation of domainspecific hydraulic parameters. The parameters were subsequently used for performance evaluation of an equivalent continuum model with bimodal hydraulic functions (ECM) and two dual-permeability models with first-order (DPM1) and second-order (DPM2) terms for water transfer between macropore and matrix. Overall, DPM2 gave slightly more accurate domain-specific results of water flow, water contents, and pressure heads than DPM1. Although bulk soil water flow results for the ECM were least accurate, they were considered to be within acceptable range. Compared to the more comprehensive ARE approach, DPMs were found to be almost equally capable of simulating interdomain and intradomain water flow and could be considered more versatile as they allow for a variety of macropore-matrix geometries.Citation: Köhne, J. M., and B. P. Mohanty (2005), Water flow processes in a soil column with a cylindrical macropore: Experiment and hierarchical modeling, Water Resour. Res., 41, W03010,

Knowledge of soil hydraulic parameters and their spatiotemporal variation is crucial for estimating the water and solute fluxes across the land-atmosphere boundary and within the vadose zone at different scales. The objective of this study was to determine soil hydraulic conductivities [saturated hydraulic conductivity, K sat , and unsaturated hydraulic conductivity, K(Y)] and their spatial and temporal variations in a clay-dominated biporous Vertisol near College Station, TX, using tension infiltrometers. The study was conducted within a 20-by 16-m plot across several seasons during a 21-mo period (May 2003-January 2005) to investigate the impact of varying disk sizes (measurement support) on K(Y), and the spatial and temporal variations of K(Y) under natural environmental conditions due to pore space evolution. Infiltration occurred in a bimodal fashion consisting of preferential flow (occurring at soil water pressure heads [Y] 5 20.05 to 0 m) and matrix flow (at Y 5 20.2 to 20.1 m). Biological and structural macropores present in the soil resulted in gravity-dominated flow near saturation (Y 5 20.05 to 0 m) for all experiments. The Student's t-test of analysis of variance indicated that hydraulic conductivities were not affected by changes in the infiltration disk sizes. Although the K(Y) values at four different locations within the plot did not show significant spatial variability, they demonstrated strong temporal variation during the 21-mo period based on the evolution of natural environmental conditions due to seasonal precipitation, root growth and decay, and structural pore space dynamics. Temporal trends of K(Y) indicated that hydraulic conductivities close to saturation were positively correlated with antecedent moisture conditions reflecting liquid cohesion, water films bridging across cracking peds, and the activation of flow in biological and structural macroporosity in the biporous soil system.