Green and Ampt analyses for rising water table and intermittent surface flux conditions

Abstract

Equations developed for a Green and Ampt type analysis for moving water table conditions in an initially saturated homogeneous profile of porous material and used previously in a falling water table study are now applied to a falling‐rising water table sequence in a sand. The ability of the developed equations to be used in a system involving a directional change in water table movement relates to the inclusion in the equations of a term describing the initial saturated depth applicable to each new stage of the analysis. The predictive capability of the Green and Ampt approach for such a system is tested by comparing the results with those obtained from a computer‐based numerical analysis. The relevant subroutine in the computer program required certain restructuring to accommodate the rising water table component. An interpolative hysteresis model is used to describe the accompanying wetting process with the material in question wetting up along primary wetting scanning curves. In comparison with the complexity of the real system during rewetting, the simplicity of the Green and Ampt approach is very marked. Notwithstanding this, it is found that reasonable correspondence occurs, although care is necessary in specifying an appropriate value, applicable to the wetting process, for the characteristic pressure head at the sharp front. The Green and Ampt approach is also extended to include a falling water table system where an applied surface flux of constant magnitude is discontinued during the gravity drainage process. The analysis can be readily adapted to such intermittent inflow and, again, comparisons with a numerical simulation indicate that satisfactory accuracy is achievable.