Climatically, the Nebraska Sand Hills are situated in a semi-arid region where evapotranspiration exceeds precipitation throughout most of the year. However, the high infiltration rates of the sandy soils has led to the accumulation of a large groundwater reservoir beneath this region. When the water table is at or near the surface, lakes, marshes and subirrigated meadows are formed. The presence of these wetland areas significantly increases the rate of evapotranspiration from the Sand Hills

The most recent version of MM5 includes a detailed land-surface model that represents a significant improvement in how MM5 computes surface fluxes and allows more realistic investigations of the effects of land surface characteristics on the atmosphere. Specifically, soil hydrology is now included explicitly, which vastly improves the utility of MM5 for hydrologic modeling. However, in the present form of the OSU land-surface model used in MM5, there is no provision for groundwater to interact with the atmosphere. In order to investigate the possible impact of groundwater on evapotranspiration, the lowest soil layer in the MM5/OSU land surface model was kept saturated for all grid points within the Sand Hills. Upward diffusion of moisture from this saturated zone is then available for withdrawal by plant roots and will contribute to regional evapotranspiration. The results of these sensitivity experiments are analyzed and presented.