A TRANSFORMED PRESSURE HEAD-BASED APPROACH TO SOLVE RICHARDS EQUATION FOR VARIABLY SATURATED SOILS

This paper presents a new approach to solve Richards' equation. It introduces a nonlinear transformed pressure, P-t, as the dependent variable with the modified Picard method. The new approach was compared to, and contrasted with, two efficient existing methods: the phi-based transformation method (Kirkland et al., 1992), and the h-based modified Picard method (Celia et al., 1990). A total of 12 different one-dimensional cases were considered (saturated, unsaturated, layered and uniform soil profiles, with pressure and flux type boundary conditions). The results show that the new method offers excellent CPU efficiency and, unlike the h-based method, is numerically robust for all cases of variably saturated, heterogeneous media, and first or second type boundary conditions. The method does not require difficult numerical coding, and its CPU efficiency is not affected by complicated heterogeneous and hysteretic media. The P-t transformation is easy to incorporate into existing h-based codes.