RICHARDS - COMPUTER-PROGRAM FOR THE NUMERICAL-SIMULATION OF ONE-DIMENSIONAL INFILTRATION INTO UNSATURATED SOIL

Flow into unsaturated soil is governed by the Richards partial differential equation expressing the mass conservation law for the water. Numerical approximations based on different forms of this one-dimensional (1-D) nonlinear partial differential equation can lead to significantly different results for unsaturated flow problems. The Richards equation may be written in three standard forms: the ''h-based'' form, the ''theta-based'' form, and the ''mixed'' form. Documentation is presented for a program which integrates the three standard forms of this equation using finite difference and finite-element techniques. Boundary conditions at prefixed pressure head and prefixed injection rate are allowed: moreover, mass balance errors are computed for controlling the accuracy of the results and for comparing the performances of the three standard forms. The listing of the computer code written in FORTRAN 77 is included along with the user manual and some illustrative infiltration tests in homogeneous and in layered soils.