EVALUATION OF A DISTRIBUTED CATCHMENT SCALE WATER-BALANCE MODEL

The validity of some of the simplifying assumptions in a conceptual wa ter balance model is investigated by comparing simulation results from the conceptual model with simulation results from a three-dimensional physically based numerical model and with field observations. We exam ine, in particular, assumptions and simplifications related to water t able dynamics, vertical soil moisture and pressure head distributions, and subsurface flow contributions to stream discharge. The conceptual model relies on a topographic index to predict saturation excess runo ff and on Philip's infiltration equation to predict infiltration exces s runoff. The numerical model solves the three-dimensional Richards eq uation describing flow in variably saturated porous media, and handles seepage face boundaries, infiltration excess and saturation excess ru noff production, and soil driven and atmosphere driven surface fluxes. The study catchments (a 7.2-km2 catchment and a 0.64-km2 subcatchment ) are located in the North Appalachian ridge and valley region of east ern Pennsylvania. Hydrologic data collected during the MACHYDRO 90 fie ld experiment are used to calibrate the models and to evaluate simulat ion results. It is found that water table dynamics as predicted by the conceptual model are close to the observations in a shallow water wel l and therefore, that a linear relationship between a topographic inde x and the local water table depth is found to be a reasonable assumpti on for catchment scale modeling. However, the hydraulic equilibrium as sumption is not valid for the upper 100 cm layer of the unsaturated zo ne and a conceptual model that incorporates a root zone is suggested. Furthermore, theoretical subsurface flow characteristics from the conc eptual model are found to be different from field observations, numeri cal simulation results, and theoretical base recession characteristics based on Boussinesq's groundwater equation.