Appraisal of the water resources of the Big Sioux Aquifer, Moody County, South Dakota

The Big Sioux aquifer in Moody County is a 55 sq mi, water table aquifer hydraulically connected to the Big Sioux River. The average thickness is 22 ft and the maximum thickness is 54 ft. A digital model was developed to simulate groundwater flow in the Big Sioux aquifer in Moody County. The model was calibrated for steady-state conditions using average hydrologic conditions from 1970 through 1979. Steady-state simulated water levels from 11 wells averaged 0.4 ft higher than measured water levels. The model was calibrated for transient conditions using 1983 water levels. The average monthly difference in 27 observation wells between simulated and measured water levels was 2.63 ft. Sensitivity analyses showed that recharge rate and evapotranspiration extinction depth had the largest effect on simulated water levels. A 4-inch/year increase in the recharge rate caused simulated water levels to rise 1.7 ft. A 2.5-ft increase in the extinction depth caused simulated water levels to decline 0.8 ft. The calibrated model was used to simulate the effects of three hypothetical hydrologic situations. The first situation simulated the transient effects of 1983 pumpage under severe drought conditions. The second hypothetical situation simulated the steady-state effects of increased pumping at a rate of 5,200 acre-ft/year under average hydrologic conditions. The third hypothetical situation simulated the transient effects of pumping 5,200 acre-ft/year under severe drought conditions. Stored water was not depleted in any nodes after these simulations. (USGS)