Appraisal of the water resources of the Big Sioux aquifer, Brookings, Deuel, and Hamlin Counties, South Dakota

A finite-difference method digital model was used to simulate steady-state conditions of the Big Sioux aquifer in Brookings, Deuel, and Hamlin Counties, S. Dak. Average annual water levels in the Big Sioux aquifer and average base flow discharge 58 cubic feet per second on the Big Sioux River near Brookings were based on the period 1970 through 1976. The computer model was used to model transient conditions by simulating monthly periods from April through August 1976. Evapotranspiration and pumpage changes were made for each month. A computer simulation was made without irrigation pumpage which resulted in an increase in the base flow from 0.66 to 9 cubic feet per second for August 1976 in the Big Sioux River near Brookings. Two transient simulations , one with the drought conditions of 1976 and one using all the pumpage allowed by irrigation permits approved by the State as of February 1979 showed, as a result of pumpage, that there was a decrease in evapotranspiration and a decrease in discharge to streams which amounts to 26 and 31% of the total groundwater pumped. Groundwater and surface water in the study area are primarily calcium bicarbonate types and are chemically suitable for irrigation with respect to sodium hazard. Sepcific conductance of groundwater ranged from 407 to 1,790 micromhos per centimeter at 25 Celsius. (USGS)