ROBUST SOLUTION OF RICHARDS EQUATION FOR NONUNIFORM POROUS-MEDIA

Capillary pressure-saturation-relative permeability relations describe d using the van Genuchten [1980] and Mualem [1976] models for nonunifo rm porous media lead to numerical convergence difficulties when used w ith Richards' equation for certain auxiliary conditions. These difficu lties arise because of discontinuities in the derivative of specific m oisture capacity and relative permeability as a function of capillary pressure. Convergence difficulties are illustrated using standard nume rical approaches to simulate such problems. We investigate constitutiv e relations: interblock permeability, nonlinear algebraic system appro ximation methods, and two time integration approaches. An integral per meability approach approximated by Hermite polynomials is recommended and shown to be robust and economical for a set of test problems, whic h correspond to sand, loam, and clay loam media.