AN EFFICIENT METHOD FOR SIMULATING STEADY UNSATURATED FLOW IN RANDOM POROUS-MEDIA - USING AN ANALYTICAL PERTURBATION SOLUTION AS INITIAL GUESS TO A NUMERICAL-MODEL

Numerical simulation of flow through multidimensional heterogeneous so ils under unsaturated conditions is a computationally intensive task. The governing unsaturated flow equation is nonlinear. The degree of no nlinearity depends on the unsaturated hydraulic properties of the soil and the degree of heterogeneity. In this paper, we present a highly e fficient method to simulate unsaturated steady state flow through rand om porous media. Our method utilizes the analytical (approximate) solu tion derived by a perturbation-spectral method as an initial guess sol ution for a numerical model to simulate two-dimensional vertical infil tration problems. It is found that this approach, which we call ''ASIG Ning,'' reduces the required CPU time by one to two orders of magnitud e. ASIGNing is demonstrated to operate successfully under a wide varie ty of boundary conditions which may substantially deviate from those i mposed on the initial guess solution. A large range of mean and varian ces in the independent variables ln K(s) and alpha or alternatively ln alpha has been tested and it is shown that the method works well for variances of the unsaturated hydraulic conductivity sigma(ln K')2 less -than-or-equal-to 5 and average [alpha] less-than-or-equal-to 0.1 [cm- 1].