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An evaluation of the relation between pumpage and change in storage was conducted for most of a three-county area in southwestern Nebraska from 1975 through 1983. Initial comparison of the 1975-83 pumpage with change in storage in the study area indicated that the 1,042,300 acre-feet of change in storage was only about 30 percent of the 3,425,000 acre-feet of pumpage. An evaluation of the data used to calculate pumpage and change in storage indicated that there was a relatively large potential for error in estimates of specific yield. As a result, minimum and maximum values of specific yield were estimated and used to recalculate change in storage. Estimates also were derived for the minimum and maximum amounts of recharge that could occur as a result of cultivation practices. The minimum and maximum estimates for specific yield and for recharge from cultivation practices were used to compute a range of values for the potential amount of additional recharge that occurred as a result of irrigation. The minimum and maximum amounts of recharge that could be caused by irrigation in the study area were 953,200 acre-feet (28 percent of pumpage) and 2,611,200 acre-feet (76 percent of pumpage), respectively. These values indicate that a substantial percentage of the water pumped from the aquifer is resupplied to storage in the aquifer as a result of a combination of irrigation returnflow and enhanced recharge from precipitation that results from cultivation and irrigation practices. INTRODUCTION The U.S. Geological Survey began a study of the High Plains aquifer (fig. 1) in 1978 as part of the Regional Aquifer-System Analysis (RASA) program. The purposes of the High Plains RASA were to provide hydrologic information needed to evaluate the effects of continued ground-water development, and to develop computer models to predict aquifer response to changes in ground-water development. During the study, the volume of ground water pumped (pumpage) and the change in the volume of water stored in the aquifer (change in storage) were determined for the period from predevelopment through 1980 (Heimes and Luckey, 1982, and Gutentag and others, 1984). A comparison of the values determined from predevelopment through 1980 indicated that the estimated change in volume in storage amounted to only about 37 percent of the volume pumped. The large difference between pumpage and change in storage indicated that additional study was required to define the factors affecting the water budget for irrigated areas of the High Plains.

water-resources investigations report

Comparison of irrigation pumpage with change in ground-water storage in the High Plains aquifer in Chase, Dundy, and Perkins counties, Nebraska, 1975-83