Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

Abstract

In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000) 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile–immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a) 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994) 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various non-equilibrium flow and transport models are also provided.