PC-Progress - Version Hydrus-DualPerm -Dual-Permeability Module Software

Warning: This module is usually significantly less numerically stable than the regular HYDRUS code due to much larger nonlinearity usually associated with the fracture domain and it therefore requires much finer spatial discretization then the standard HYDRUS. It is thus intended mainly for more experienced HYDRUS users!

Preferential flow in structured media (both macroporous soils and fractured rocks) can be described using a variety of dual-porosity, dual-permeability, multi-porosity, and/or multi-permeability models. Dual-porosity and Dual-Permeability models both assume that the porous medium consists of two interacting regions, one associated with the inter-aggregate, macropore, or fracture system, and one comprising micropores (or intra-aggregate pores) inside soil aggregates or the rock matrix. While dual-porosity models assume that water in the matrix is stagnant, dual-permeability models allow for water flow in the matrix as well. While dual-porosity models are available in the standard version of HYDRUS (2D/3D), dual-permeability models are not.

Dual-porosity models have long been applied to solute transport studies. Especially popular early on were dual-porosity models in which distinct mobile and immobile flow regions are assumed to be present. Dual-permeability models in which water can move in both the inter- and intra-aggregate pore regions are now also becoming more popular. Available dual-permeability models differ mainly in how they implement water flow in and between the two pore regions, especially with respect to the degree of simplification and empiricism. Approaches to calculating water flow in macropores or inter-aggregate pores range from those invoking Poiseuille’s equation, the Green and Ampt or Philip infiltration models, the kinematic wave equation, and the Richards equation (Gerke and van Genuchten, 1993a).

We have implemented the dual-permeability model based on the approach suggested by Gerke and van Genuchten (1993a). The dual-permeability formulation for water flow is based on a mixed form of the Richards equation, describing water flow in both the fractures (macropores) and the matrix (micropores) domains [Gerke and van Genuchten, 1993a]. The dual-permeability formulation for solute transport is based on a convection-dispersion equation, describing solute transport in both the fractures (macropores) and the matrix (micropores) domains [Gerke and van Genuchten, 1993a]. The mass transfer of water between the two domains is driven by the gradient of pressure heads. The mass transfer for solute includes both convective mass transfer with water mass transfer, as well as diffusive mass transfer driven by the concentration gradient. See the detailed description of this model either in references given below or in the DualPerm Module manual.