Eect of the shape of the soil hydraulic functions near saturation on variably-saturated ̄ow predictions

Relatively small changes in the shape of the soil water retention curve near saturation can signi®cantly a€ect the results of numerical simulations of variably saturated ̄ow, including the performance of the numerical scheme itself in terms of stability and rate of convergence. In this paper, we use a modi®ed form of the van Genuchten±Mualem (VGM) soil hydraulic functions to account for a very small, but non-zero minimum capillary height, hs, in the soil water retention curve. The modi®ed VGM model is contrasted with the original formulation by comparing simulation results for in®ltration in homogeneous soils assuming both constant pressure and constant ̄ux boundary conditions. The two models gave signi®cantly di€erent results for in®ltration in ®netextured soils, even for hs-values as small as )1 cm. Incorporating a small minimum capillary height in the hydraulic properties leads to less non-linearity in the hydraulic conductivity function near saturation and, because of this, to more stable numerical solutions of the ̄ow equation. This study indicates an urgent need for experimental studies that assess the precise shape of the hydraulic conductivity curve near saturation, especially for relatively ®ne-textured soils. For one example we found considerable improvement in the predicted conductivity function when a value of )2 cm for hs was used in the modi®ed VGM model. Ó 2000 Elsevier Science Ltd. All rights reserved.