COMPARISON OF THE EAGLESON STATISTICAL-DYNAMICAL WATER-BALANCE MODEL WITH NUMERICAL SIMULATIONS

With minor modification an existing analytic model employing a simple statistical description of soil moisture dynamics is found to give acc urate estimates of the mean water balance partitioning; comparisons ar e made to a Monte Carlo numerical simulation of unsaturated moisture f low in a soil column. In the statistical-dynamical model the complex t emporal and spatial characteristics of the soil moisture profile are a pproximated by a single effective soil moisture: the equilibrium soil moisture (s0) for which the modeled long-term mean fluxes are in balan ce. The relation between this equilibrium soil moisture and the equiva lent steady state moisture profile (the steady profile which transmits the long-term mean flow) is discussed. The test numerical integration s of the flow and conservation equations are designed to represent a d eep one-dimensional soil column. Soil surface boundary conditions are defined by the interaction of the surface soil moisture with a stochas tic event-based atmospheric forcing model, allowing for state dependen t switching from flux boundary conditions (rainfall intensity and pote ntial evaporation) to concentration conditions (ponded and dry surface ), thus simulating infiltration-excess runoff production and soil-limi ted evaporation. The ability of the equilibrium model to estimate the mean water balance under a large range of soil textures (from clay to sand loam) and climate forcings (humid to arid) implies that the soil moisture profile may, for purposes of climate water balance, be repres ented in a simple fashion.