Appropriate vertical discretization of Richards? equation for two-dimensional watershed-scale modelling

A number of watershed-scale hydrological models include RICHARDS' equation (RE) solutions. In this study, spatial convergence studies are conducted using the two-dimensional distributed-parameters, gridded surface-subsurface hydrological analysis model which solves RE to simulate vadose zone fluxes. Tests to determine if the required discretisation is strongly a function of dominant runoff production mechanism are conducted using data from two different watersheds. Results indicate that to simulate hydrological fluxes accurately small vertical cell sizes are required near the soil surface.