Ground-water discharge to Willow Creek, which drains a 204 mi² watershed in northeast Nebraska was predicted for six combinations of conditions of climate and ground-water development. A digital model of the ground water/surface-water system was stressed with recharge and withdrawal functions determined from a linear reservoir model of the soil zone.

The geohydrologic system includes a regional aquifer which containssand and gravel ranging in age from Pliocene through Pleistocene. The regional aquifer is unconfined in the western part of the watershed and confined in the eastern part. The confining layer, or blue clay, in the east is of Pleistocene age and comprises principally saturated eolian silts with very fine sand interbeds overlying a basal lacustrine clay. Where unconfined conditions exist in the regional aquifer, perennial flow of Willow Creek is sustained principally by ground-water discharge. Where confined conditions exist, the low hydraulic conductivity of the blue clay effectively isolates the regional aquifer from Willow Creek.

The stream-aquifer models were tested by comparing simulated against measured water levels and streamflow during 1975 and 1976. Agreement between simulated and observed values was obtained by modifying initial estimates of aquifer hydraulic conductivity and specific storage.

Future ground-water discharge to Willow Creek (base flow) was simulated by super-imposing different patterns of ground-water withdrawals upon variations from average recharge for a monthly climatic sequence identical with the period 1931 to 1954. Frequency analyses of the simulated monthly base flows showed that they would be less than 12 feet³ per second at least 50 percent of the time under all but completely undeveloped conditions. Simulated water-level declines in the regional aquifer at the end of the pumping season after a drought period similar to that of the 1930's were less than 5 feet where unconfined conditions exist and were more than 80 feet where confined conditions exist.