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, eva potranspiration, net recharge, and groundwater divergence are constrained b y mutual dependence on the shape and position of the water table. The water table position impacts the partitioning of rainfall by bounding the moistu re profile at depth and creating a potential source of capillary rise to th e root zone. By coupling a water table dependent vadose zone model that tim e 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 fl ow 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 inf iltration. Parameters of the coupled model include soil physical properties (permeability, porosity, tension-saturated matric potential! and Brooks an d Corey pore size distribution index), basin topography, aquifer hydraulic conductivity, and statistical moments of the probability distributions of m eteorological variables (storm intensity, storm duration, time between stor ms, 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 zon e interactions and lateral groundwater redistribution in watershed modeling .