The simulation of soil moisture and moisture fluxes
in the unsaturated zone is performed using the Atmosphere
and Landsurface Interaction Scheme (ALSIS). In
essence, ALSIS describes the soil hydrological and thermal
processes in the unsaturated zone. It was originally
designed for providing the lower boundary conditions
for a regional atmospheric model by calculating the
momentum, mass and energy fluxes at the surface. It has
been expanded recently into an interface between the
atmospheric model and a regional ground-water model
by including the calculation of drainage from the unsaturated
zone to the ground-water system and allowing spatial
and temporal variations of the ground-water table.
A detailed description of ALSIS and its validation with
observed data are given by Irannejad and Shao (1998).
In the following, its hydrological component is outlined.
Consider a 1-D isothermal Darcian flow in a variablysaturated
rigid porous medium and assume that the fluid
is incompressible and the air phase plays an insignificant
role in the water flow. The continuity equation governing
the water flow for these conditions is given by the following
modified form of the Richards equation