Geohydrology of the regional aquifer system, western Snake River plain, southwestern Idaho

A three dimensional groundwater flow model was developed to simulate steady state and nonsteady-state hydrologic conditions of the regional aquifer system in the western Snake River Plain of Idaho. Water budget analysis showed that groundwater recharge was about 1,400,000 acre-ft in 1980; groundwater pumpage was estimated to be 300,000 acre-ft. Two mass water level measurements were made in March and August 1980 to define the water table in the regional system. The model was discretized into 25 rows, 72 columns, and 3 layers. Each cell represented 4 sq mi. The model was calibrated to 1980 hydrologic conditions. Calibrated transmissivity of layer 1 (500 ft thick) ranged from 1,500 to 21,500 sq ft/day. Calibrated specific yield of unconfined aquifers was 0.10 and calibrated storage coefficient of confined aquifers ranged from 0.0004 to 0.007. The calibrated model was verified by simulating monthly water-level fluctuations for 1980. Simulated water levels matched measured levels in the Boise River Valley, but the match in other areas was poor. (USGS)