Modeling flow and transport in a two-dimensional dual-permeability system with spatially variable hydraulic properties

Abstract

Most field soils exhibit soil spatial variability as well as soil structure. The challenge is to account adequately for both types of spatial heterogeneity in simulation models. A numerical finite element code was used to compare single- and dual-permeability approaches for modeling variably saturated flow and transport in two-dimensional heterogeneous soil systems. The code was based on the Richards' equation for water flow and the advection-dispersion equation for solute transport. Spatial variability in the soil hydraulic properties was accounted for by randomly generating a hydraulic conductivity field using a one-dimensional first-order Markov process. Soil structural effects were modeled with a two-domain concept in which a first-order kinetic expression is used to describe the transfer of water and solute between the two domains. Numerical experiments were carried out for the case of furrow irrigation, including the breakthrough of a conservative solute to the groundwater table. We compared five different scenarios: a single domain having uniform hydraulic properties (SU), a single domain with a randomly distributed hydraulic conductivity (SR), a dual-permeability system with uniform hydraulic properties (DU), a dual-permeability system with a randomly distributed fracture hydraulic conductivity (DRF), and a dual-permeability system having a randomly distributed matrix hydraulic conductivity (DRM). All scenarios started with pressure heads in equilibrium with a constant groundwater table 150 cm below the soil surface and zero initial solute concentrations. The simulated two-dimensional (2D) vertical concentration profiles showed preferential pathways resulting from both the spatial variability (SR) and soil structure (DRF) scenarios. As expected, drainage of water from the bottom of the profile occurred significantly earlier for dual-than for single-permeability scenarios. The combination of having spatial variability in the hydraulic properties and invoking the dual-permeability approach yielded the quickest and largest leaching of solute. The 2D dual-permeability approach should considerably improve the simulation of water and solute movement in naturally heterogeneous field soils.