Simulation of mildly unsaturated flow

In the study of unsaturated flow in the soil profile, the soil characteristic functions, one relating the pressure head to water saturation and the other relating the permeability to water saturation, are usually established with data covering a wide range. The application of these functions to situations with a mildly unsaturated range, such as encountered in a wet irrigation setting in a coarse soil, has been investigated. Certain shortcomings in using these soil characteristic functions directly have been identified, and modifications to the procedure of determining the characteristic functions have been proposed and tested. The improvements have been quantified in terms of the goodness of fit to published field measurements on the soil characteristics and also in terms of simulation results for an idealized test situation corresponding to irrigation practice in a relatively coarse soil. The special features of the C language have been utilized in developing a computer program for finite element modelling of the nonlinear Richards' equation describing unsaturated subsurface water flow. A weighted least-squares procedure improves the fit of the Brooks and Corey characteristic functions for pressure head vs. degree of saturation. A numerical integration procedure improves the fit of the Van Genuchten characteristic functions for permeability vs. degree of saturation. The present study indicates that even for sandy soil, the conventional Van Genuchten and Brooks and Corey soil characteristic functions need to be modified to yield acceptable results for the mildly unsaturated regime. © 1994.