A COUPLED LANDSCAPE AND GROUNDWATER-FLOW MODEL OF THE NORTHERN HIGH PLAINS AQUIFER

The High Plains aquifer, primarily composed of the Ogallala Formation of Tertiary age, underlies about 174,000 square miles of eight states from South Dakota to Texas. Groundwater pumped from the High Plains aquifer is used primarily for irrigated agriculture, and accounts for 20 percent of all groundwater withdrawn in the United States. However, water-level declines in some areas of the aquifer have caused concerns about long-term sustainability of the groundwater resource and potential negative consequences to National agricultural production.

The High Plains Groundwater Availability Study, one of series of regional studies, was initiated by the U.S. Geological Survey’s Groundwater Resources Program to provide an updated assessment of the major components of the water budget of the High Plains aquifer and their interaction, and to provide information to help manage and protect the resource. A key component of the study is a new model for the northern High Plains aquifer (the High Plains aquifer north of the Smoky Hill River), simulating landscape hydrology fully coupled with groundwater flow. For example, simulated landscape hydrologic processes include precipitation, evapotranspiration, runoff, surface-water irrigation application and conveyance, and recharge. These landscape hydrologic processes, along with simulated direct uptake of groundwater, in turn determine the groundwater withdrawals for irrigation. Conversely, simulated groundwater levels affect the rate of direct uptake of groundwater, and simulated groundwater discharge to streams affects the amount of routed streamflow that is available for diversion for surface-water irrigation. The fully-coupled landscape and groundwater simulation allows for more complete representation of all hydrologic components from both natural and anthropogenic causes. Further, the new groundwater-flow model is developed at a finer spatial and temporal resolution than was possible during previous U.S. Geological Survey studies of the northern High Plains aquifer. The new northern High Plains model also integrates data and understanding contributed by the many studies of smaller parts of the study area that have occurred in recent decades.