Equilibrium analysis of groundwater-vadose zone interactions and the resulting spatial distribution of hydrologic fluxes across a Canadian prairie

The spatial distribution and magnitude of time-averaged surface runoff, evapotranspiration, net recharge, and groundwater divergence are constrained by mutual dependence on the shape and position of the water table. The water table position impacts the partitioning of rainfall by bounding the moisture profile at depth and creating a potential source of capillary rise to the root zone. By coupling a water table dependent vadose zone model that time averages over event-scale surface fluxes to a regional groundwater model (MODFLOW-96), we are able (1) to estimate the unique shape and position of the water table for which net recharge (percolation additions or capillary rise losses) is balanced by the divergence of the underlying groundwater flow field, (2) to estimate the distribution of recharge and discharge to and from an aquifer, and (3) to estimate the spatial distribution of the long-term mean partitioning of rainfall into evapotranspiration, runoff, and infiltration. Parameters of the coupled model include soil physical properties (permeability, porosity, tension-saturated matric potential! and Brooks and Corey pore size distribution index), basin topography, aquifer hydraulic conductivity, and statistical moments of the probability distributions of meteorological variables (storm intensity, storm duration, time between storms, and potential evaporation). The recharge and discharge areas predicted by the equilibrium model (without specifying a priori the water table shape) are in good agreement with field estimates by Toth [1966] for a Canadian prairie, indicating the importance of accounting for groundwater-vadose zone interactions and lateral groundwater redistribution in watershed modeling.