The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District (NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model (ELM) area using a ground-water-flow model. The ELM area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow model was constructed with a single layer vertically to represent the aquifers of the Tertiary-age Ogallala Group and Quaternary-age alluvial deposits, with a uniform node spacing of 2 miles. The model will be calibrated to measured ground-water levels and estimated ground-water discharge to streams for the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 period will be calibrated to measure ground-water level changes. The study results will assist Nebraska Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for managing hydrologically connected waters.

This dataset is one of three geospatial datasets that together revise previously published maps of the configuration of the base of the principal aquifer and of the geologic units that form the base of the principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is an unnamed alluvial aquifer.

This dataset identifies and delimits approximately the extent of the geologic units forming the base of the principal aquifer and the areas where the base of principal aquifer surface is not well defined. Ambiguity in the calculated base of aquifer existed wherever this calculated base-of-aquifer altitude was above the mapped land surface. The calculated base of aquifer was above the mapped land surface either because of inaccuracy in the mapped land surface or base of aquifer calculation or because the principal aquifer was thin or not present.