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 and large storage capacity of the underlying sediments 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 these regions of the Sand Hills from what would be expected climatically.

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. One-dimensional simulations with the OSU soil scheme, driven by observed atmospheric and soil conditions, have demonstrated the impact of groundwater inflow on the simulated soil moisture of a subirrigated meadow at the Gudmundsen Sandhills Laboratory in the northwestern Sand Hills. When the model is run without groundwater input, simulated soil moisture in the top meter is considerably underestimated. However, when groundwater is incorporated into the simulations, the resulting soil moisture levels correspond well to the observations.

The potential impact of near-surface groundwater on regional evapotranspiration is investigated by maintaining the lowest soil layers in the MM5/OSU land surface model at saturation for grid points within the Sand Hills that represent subirrigated meadows and wetlands. Extraction of moisture from the saturated zone by plant roots enhances regional evapotranspiration and modifies the atmospheric boundary layer.