The Nebraska Sand Hills are the largest sand dune area in the Western Hemisphere and one of the largest grass-stabilized dune areas in the world. Even though the Sand Hills are generally perceived as a dry environment, they play an important role in the regional hydrology as a significant source of both groundwater recharge and surface flow. With a climate varying from subhumid in the east to semiarid in its western portion, the Sand Hills supports a unique mix of vegetation that changes across its breadth. In addition, the highly permeable sand dunes have allowed the accumulation of a substantial groundwater reservoir situated below the dry dune tops, but often near or even above the floor of the interdunal valleys with their marshes and lakes. Furthermore, the sandy soils of the Sand Hills, with their high infiltration rates, form a clear contrast to the soils in the cropped areas south of the Sand Hills in southern and southeastern Nebraska and extending into Kansas and Missouri. In these regions, clay type soils with low hydraulic conductivity and slow infiltration rate predominate. The resulting spatial heterogeneity, both within the Sand Hills and between the region and its surroundings, presents a challenge to understanding the role of land-atmosphere interactions and their seasonal variations on the region's weather and its hydrological cycle.

The major thrust of this project is to understand how the spatially heterogeneous and temporally variable landscape of the Nebraska Sand Hills affects the regional surface and groundwater hydrology and what role local land surface-atmosphere interactions play in the regional hydrological system. Atmospheric and groundwater models are being used to acquire a better understanding of the hydrological cycle in the Nebraska Sand Hills and to gain insight into land surface-atmosphere interactions over this region. Separate simulations have been made with a groundwater model and an atmospheric mesoscale model for wet, dry and near-normal periods which allows comparisons of the moisture fluxes at the interface of their respective domains. Two separate comparisons may be made: 1) drainage from the lowest soil layer of the atmospheric model can be compared to the recharge input to the groundwater model computed from observed precipitation and 2) evapotranspiration from the groundwater model can be compared to the assumption of no upward moisture flux from below the lowest soil layer in the atmospheric model. These comparisons provide an estimate of the potential error involved in the assumptions of each model.