Numerical analysis of vertical water movement in a bounded profile

Abstract

The usual assumption made in soil water studies that the effect of the air phase on the flow process is negligible is discussed in relation to certain profile configurations where such an assumption is not valid. A computer-based numerical solution of the equation describing water movement in an unsaturated soil is then modified by the inclusion of a time-dependent boundary condition, thus allowing the modelling of the air compression effect during infiltration. This analysis also satisfactorily models the drainage along primary draining scanning curves which occurs in the upper part of the profile as the air pressure increases. The analysis is limited to the Darcy flow regime which, for the system considered, terminates when the excess air pressure equals the air entry value of the porous material. Detailed results are presented for a sand and a sandy loam. Profile depths of 200 cm and 1000 cm have been analysed, and comparisons made of the effect of the air compression on the shape of the wetting front. The effect on cumulative infiltration is also noted.