Use of the Richards equation in land surface parameterizations

Abstract

Accurately modeling infiltration and soil moisture within land surface parameterization schemes (LSPs) of coupled land surface‐atmosphere models is essential for producing realistic simulations of energy and moisture fluxes and for partitioning precipitation into infiltration, surface runoff, and drainage to groundwater. This report compares simulations of soil moisture, runoff, infiltration, and drainage to groundwater for a bare clay loam using three approaches: a finite difference solution of the vertically integrated Richards equation (an approach commonly used in LSPs), a highly resolved (spatially and temporally) finite element solution of Richards equation, and an analytical kinematic wave solution of Richards equation. Comparisons show that depth‐averaged soil moisture simulated using the vertically integrated Richards equation is only similar to those of the finite element solution for vertical spatial discretizations finer than those employed by most state‐of‐the‐art land surface‐atmosphere transfer schemes. The vertically integrated Richards equation overpredicts soil moisture in the near‐surface soil column and underpredicts drainage to groundwater. The infiltration formulation is found to be critical in partitioning precipitation into runoff, soil moisture, and drainage. Different infiltration formulations and vertical spatial discretizations may partly explain the very different land surface moisture and energy fluxes reported by the LSPs evaluated as part of the Project for Intercomparison of Land Surface Parameterization Schemes (PILPS) Phase 2(b) experiment.