To determine the hydraulic properties of soils, transient flow experiments on soil samples and their evaluation by parameter identification methods have become common. In the past, the feasibility of this method has been investigated for soils exclusively with unimodal poresize distributions. In this work we investigated, both from a theoretical and from an experimental point of view, whether hydraulic properties of structured soils with bimodal pore-size characteristics can be identified by inverse simulation. Multistep outflow experiments were simulated for three hypothetical soils with different degrees of bimodality. The outflow data thus created were then used in the inverse procedure, both in the original form and with an added random error. An analysis of sensitivity coefficients for the model parameters showed that if the bimodality of the pore system is well developed, the parameters of the bimodal hydraulic functions are uncorrelated and can be identified. This was confirmed by inverse optimization runs with simultaneous optimization of up to six parameters, which consistently converged from different initial values to the true parameter values, indicating uniqueness of the inverse problem. Attempts to fit the bimodal outflow data with simulations using unimodal hydraulic functions were not successful, as expressed by systematic disagreement between fitted and "observed" data and nonuniqueness of the inverse solution. We validated these theoretical results with experimental data from an undisturbed sandy forest soil, which showed that it is possible to determine the retention parameters of bimodal hydraulic properties by inverse modeling of multistep outflow experiments.

In this study we investigate numerical simulations of one-dimensional water flow and solute transport in a soil with a nonuniform pore-size distribution. Water transport was modeled by treating the soil as one domain by applying Richards equation, while using alternatively a unimodal and a bimodal model for the hydraulic properties. The retention curves were fitted to a set of measured data; the relative conductivity functions were estimated by Mualem's [1976] model. Contrary to the unimodal case, the bimodal conductivity curve shows a steep decrease in water content a near saturation. Simulated water regimes under transient boundary conditions differed strongly for the two cases. The use of the bimodal functions yielded a preferential flow characteristic which was not obtained using unimodal functions. For both hydraulic regimes we modeled solute transport comparing four different variants of the convection-dispersion equation. For the classical one-region model we found that the breakthrough curve of an ideal tracer was not affected by the dynamics of the water flow. For the two-region approach, where the water-filled pore domain is divided into a mobile region am and an immobile region aim, three different conceptual treatments of am under transient conditions were investigated. For the case where aim was kept constant, the different hydraulic regimes again caused only minor differences in solute transport. The same was true for the alternative case where the ratio am/a was kept constant. However, for the third case, where am was treated as a dynamic variable which changes with the actual water content in a way that depends on the shape of the hydraulic conductivity function, the transport simulation based on the bimodal hydraulic model reflected enhanced preferential transport at high-infiltration rates.matical models.To model water transport, a proper knowledge of the soil hydraulic properties, namely, the water retention characteristic and the hydraulic conductivity function, is required. These relationships are generally expressed as simple closed-form functions, where a small number of parameters characterize the shape of the hydraulic relationships [van Genuchten and Nielsen, 1985; Mualem, 1986]. These functional descriptions generally represent pore systems with unimodal pore-size distributions [Durner, 1994]. Contrary to these simple models, natural soils often exhibit more complicated pore-size distributions, for example, with a secondary pore system in the large-pore range, which may greatly influence the hydraulic properties near saturation. In such soils the drainage of only a •Now at few percent of water causes a considerable drop in hydraulic conductivity [Carvallo et al., 1976; Parkes and Waters, 1980]. The use of unimodal functions to describe the hydraulic properties of such soils is not always adequate [Othmer et al., 1991; Durner, 1992; Wilson et al., 1992; Ross and Smettem, 1993] and may lead to considerable errors in hydraulic simulations. Since simulations of solute transport are generall...

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