The Ogallala and Arikaree aquifers are important water resources in the Rosebud Indian Reservation area and are used extensively for irrigation, municipal, and domestic water supplies. Continued or increased withdrawals from the Ogallala and Arikaree aquifers in the Rosebud Indian Reservation area have the potential to affect water levels in these aquifers. This report describes a conceptual model of ground-water flow in these aquifers and documents the development and calibration of a numerical model to simulate ground-water flow. Data for a twenty-year period (water years 1979 through 1998) were analyzed for the conceptual model and included in steady-state and transient numerical simulations of ground-water flow for the same 20-year period.

A three-dimensional ground-water flow model, with two layers, was used to simulate ground-water flow in the Ogallala and Arikaree aquifers. The upper layer represented the Ogallala aquifer, and the lower layer represented the Arikaree aquifer. The study area was divided into grid blocks 1,640 feet (500 meters) on a side, with 153 rows and 180 columns.

Areal recharge to the Ogallala and Arikaree aquifers occurs from precipitation on the outcrop areas. The recharge rate for the steady-state simulation was 3.3 inches per year for the Ogallala aquifer and 1.7 inches per year for the Arikaree aquifer for a total recharge rate of 266 cubic feet per second.

Discharge from the Ogallala and Arikaree aquifers occurs through evapotranspiration, discharge to streams, and well withdrawals. Discharge rates in cubic feet per second for the steady-state simulation were 184 for evapotranspiration, 46.8 and 19.7 for base flow to the Little White and Keya Paha Rivers, respectively, and 11.6 for well withdrawals from irrigation use. Estimated horizontal hydraulic conductivity used for the numerical model ranged from 0.2 to 120 feet per day in the Ogallala aquifer and 0.1 to 5.4 feet per day in the Arikaree aquifer. A uniform vertical hydraulic conductivity value of 6.6x10^-4 feet per day was applied to the Ogallala aquifer. Vertical hydraulic conductivity was estimated for five zones in the Arikaree aquifer and ranged from 8.6x10^-6 to 7.2x10^-1 feet per day. Average rates of recharge, maximum evapotranspiration, and well withdrawals were included in the steady-state simulation, whereas the time-varying rates were included in the transient simulation.

Model calibration was accomplished by varying parameters within plausible ranges to produce the best fit between simulated and observed hydraulic heads and base-flow discharges from the Ogallala and Arikaree aquifers. For the steady-state simulation, the root mean square error for simulated hydraulic heads for all wells was 26.8 feet. Simulated hydraulic heads were within ±50 feet of observed values for 95 percent of the wells. For the transient simulation, the difference between the simulated and observed means for hydrographs was within ±40 feet for all observation wells. The potentiometric surfaces of the two aquifers calculated by the steady-state simulation established initial conditions for the transient simulation.

A sensitivity analysis was used to examine the response of the calibrated steady-state model to changes in model parameters including horizontal and vertical hydraulic conductivity, evapotranspiration, recharge, and riverbed conductance. The model was most sensitive to recharge and horizontal hydraulic conductivity.