Numerical solution of transient flow of water in unsaturated soil with applications to outflow from soil moisture extractors

Abstract

This report presents the results of a program of numerical analysis of nonlinear partial differential equations which equations which describe nonsteady soil moisture transfer. The results, when combined with experimental data, are intended for use in determining the soil moisture transfer characteristics of various types of soil. The results of this study indicate that linear theory is significantly inaccurate in representing unsaturated transient flow of water in soils for finite perturbations. On the other hand, for step-function transient outflow experiments performed for determination of soil moisture transfer characteristics, the numerical solutions presented here allow all data obtained from the experiments to be used in characterizing the soil water diffusivity over the full range of the step. If there is a significant membrane impedance, the determination of the soil water diffusivity becomes much more complex. Because of the limited number of solutions presented, practical determinations of the diffusivity require that the membrane impedance effects be minimized. This can be done by increasing the soil slab thickness, by decreasing the pressure step size, and by working at high matric suctions. It is concluded that the effects of gravity on laboratory transient outflow experiments are negligible. /Author/