COMPARISON OF SOIL WATER INFILTRATION PROFILES OBTAINED EXPERIMENTALLY AND BY SOLUTION OF RICHARDSʼ EQUATION

We compared Hesperia and Columbia soil experimental infiltration data to calculated profiles obtained by solutions of Richards' equation requiring soil water diffusivity D and hydraulic conductivity K as functions of the water content θ. The calculations were made using a finite-difference, iterative method (FINDIT). Predicting wetting profiles by this method is somewhat more rigorous than some other solutions of Richards' equation. The method reduces calculations for infiltration to a two-term algebraic equation, partitions infiltration into matric and gravitational components, and gives an asymptotic relationship between the infiltration rate and the saturated conductivity as time approaches infinity. An array of solutions for the two indicated soils, obtained with and without the diffusion lip procedure, raises questions of the need for its use. A new method for estimating the Λ - θ area in horizontal infiltration analysis, using only diffusivity data, gives estimates to less than 3 percent of the final value. Integrated mean values of D and K were required for accurately predicting soil moisture wetting profiles over a range of time periods and θ divisions. Arithmetic and geometric means and nonaveraged values, particularly of D, produced unreliable wetting profiles.