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[1] Predictions of nonequilibrium preferential flow and transport are limited as long as the fracture and the matrix pore system cannot be represented by separate hydraulic functions. Previous approaches to describe hydraulic properties of soils with bimodal or multimodal pore size distributions are still restricted to soils with a single porous continuum. The objective of this study was to develop and test a fitting procedure for estimating retention and conductivity functions for dual-permeability models using bulk soil data. The estimation is based on a set of van Genuchten-Mualem functions for the two pore systems. A stepwise procedure is carried out that (1) assumes 3 out of 11 parameters to be known, (2) determines initial values for the remaining eight parameters to be fitted, and (3) fits the set of hydraulic functions simultaneously to q(h) and K(h) data to obtain the parameter estimates. This procedure was evaluated using a synthetic data set, and it was applied to laboratory-measured q(h) and K(h) data from a loamy Dystric Gleysol soil. The results show that the effect of a fracture pore system on the unsaturated hydraulic conductivity function might be underestimated if only observations of water retention are considered. The fitted dual-continuum hydraulic conductivity functions match the unsaturated soil's conductivity data better than a bimodal single-domain approach that predicts hydraulic conductivity based on fitted water retention functions. Moreover, the dual-continuum functions provide hydraulic parameters for dual-permeability preferential flow models.

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Dual‐permeability model simulations and sensitivity studies indicate that preferential solute leaching in structured soil is intensified by a limited solute mass transfer between preferential flow paths and matrix. It is currently unknown how much aggregate skins may affect solute mass transfer. The objective of this study was to evaluate the effect of a skin layer on the effective diffusion coefficient, De, of water saturated soil aggregates. In a diffusion experiment, the concentration decrease of Br− and Cl− in a solution being in contact with soil aggregates was measured. Aggregates with intact and removed surface skins were studied. Two different theoretical approaches were compared: (i) fitting a modified analytical solution of Crank for diffusion out of a solution of limited volume into a plane sheet to obtain De for intact and for skin‐removed aggregates; and (ii) calibrating a numerical solution for diffusion through a two‐layer system to estimate De of the skin layer and the interior part, separately. The De‐values of the skins differed for Br− and Cl− Using the first approach, De was six (Br−) and 15 (Cl−) times smaller for the intact than for the skin‐removed aggregates. The second approach resulted in effective diffusion coefficients of the skin layer being 30 (Br−) and 45 (Cl−) times smaller than those of the interior parts. Aggregate skins may reduce diffusive anion transfer between interaggregate region and soil matrix, an effect which may significantly influence preferential solute transport in structured soil.

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