Delineation of groundwater recharge, discharge and midline zones through numerical modeling and satellite radiometry

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 vadose zone model that is dependent on depth to the water table and time-averages over event-scale surface fluxes to a regional groundwater model, a new modeling framework is developed for: (1) estimation of the unique shape and position of the water table for which net recharge is balanced by the divergence of the underlying groundwater flow field; (2) estimation of the distribution of recharge and discharge to and from an aquifer; and (3) estimation of the spatial distribution of the long-term mean partitioning of rainfall into evapotranspiration, runoff and infiltration. Analysis of the relationship between stage-one (stressed) and stage-two (unstressed) transpiration shows that during stage two transpiration the bottom of the root zone behaves like a dry soil surface, and the transpiration rate is limited by the upward flux to this boundary. In addition, the duration of stage-one transpiration is controlled by the storage capacity of the root zone, the potential transpiration rate, and the net flux at the bottom of the root zone prior to stress. From these results an analytic solution for the time averaged transpiration rate is developed for use in the coupled model. The recharge and discharge areas predicted by the equilibrium model are in good agreement with published field estimates for a Canadian prairie, indicating the importance of accounting for groundwater-vadose zone interactions and lateral groundwater redistribution in watershed modeling. Also, a study of the mean and variance of surface temperature and vegetation density show them to be correlated with water table depth. This result is used to develop a remotely sensed flux index that is useful in mapping the spatial distribution of groundwater recharge and discharge areas.