A hydrogeological study including two numerical groundwater-flow models was completed for the Citizen Potawatomi Nation Tribal Jurisdictional Area of central Oklahoma. One numerical groundwater-flow model, the Citizen Potawatomi Nation model, encompassed the jurisdictional area and was based on the results of a regional-scale hydrogeological study and numerical groundwater flow model of the Central Oklahoma aquifer, which had a geographic extent that included the Citizen Potawatomi Nation Tribal Jurisdictional Area. The Citizen Potawatomi Nation numerical groundwater-flow model included alluvial aquifers not in the original model and improved calibration using automated parameter-estimation techniques. The Citizen Potawatomi Nation numerical groundwater-flow model was used to analyze the groundwater-flow system and the effects of drought on the volume of groundwater in storage and streamflow in the North Canadian River. A more detailed, local-scale inset model was constructed from the Citizen Potawatomi Nation model to estimate available groundwater resources for two Citizen Potawatomi Nation economic development zones near the North Canadian River, the geothermal supply area and the Iron Horse Industrial Park.

Groundwater pumping rates at potential well locations were optimized using the most recent version of the U.S. Geological Survey Groundwater-Management Process for MODFLOW. The objectives of optimization were to determine if a total pumping rate of 500 gallons per minute could be pumped from 5 wells at the geothermal supply area and to maximize discharge from 16 wells at the Iron Horse Industrial Park without exceeding specified head drawdown constraints at the pumping wells and thus prevent groundwater depletion.

The inset model was used to estimate North Canadian River streamflow depletion caused by optimized pumping at the Iron Horse Industrial Park because water quality was a concern, and the river may have degraded water quality compared to water in other parts of the alluvial aquifer. The fate of streamflow that infiltrates into groundwater because of pumping was not directly determined, but it was assumed that this water could end up in the well discharge, and was considered to be a maximum proportion of well discharge derived from the North Canadian River.

The total optimized continuous pumping rate from five managed wells at the geothermal supply area was 638 gallons per minute, which exceeded the target pumping rate of 500 gallons per minute. The total continuous pumping rate from 16 wells at the Iron Horse Industrial Park was 1,472 gallons per minute, which induced stream infiltration of approximately 4.1 gallons per minute (approximately 0.3 percent of the total well discharge) from the North Canadian River.

To estimate the effects of drought on water resources in the Citizen Potawatomi Nation Tribal Jurisdictional Area, a hypothetical 10-year drought during which precipitation would decrease by 50 percent was simulated by decreasing model groundwater recharge by the same proportion for the period 1990–2000 of the transient model. The effects of the drought were estimated by calculating the change in the volume of groundwater storage and groundwater flow to streams at the end of the drought period, and the change in simulated streamflow in the North Canadian River at the streamflow-gaging station at Shawnee, Okla., during and after the drought.

The hypothetical decrease in recharge during the simulated drought caused groundwater in storage over the entire model in the study area to decrease by 361,500 acre-feet (14,100 acre-feet in the North Canadian River alluvial aquifer and 347,400 acre-feet in the Central Oklahoma aquifer), or approximately 0.2 percent of the total groundwater in storage over the drought period. This small percentage of groundwater loss showed that the Central Oklahoma aquifer as a bedrock aquifer has relatively low rates of recharge from the surface relative to the approximate storage. The budget for base flow to the North Canadian River indicated that the change in groundwater flow to the North Canadian River decreased during the 10-year drought by 386,500 acre-feet, or 37 percent. In all other parts of the Citizen Potawatomi Nation Tribal Jurisdictional Area, base flow decreased by 292,000 acre-feet, or 28 percent. Streamflow in the North Canadian River at the streamflow-gaging station at Shawnee, Okla., decreased during the hypothetical drought by as much as 28 percent, and the mean change in streamflow decreased as much as 16 percent. Streamflow at the Shawnee streamflow-gaging station did not recover to nondrought conditions until about 3 years after the simulated drought ended, during the relatively wet year of 2007.