Rush Springs Aquifer Study

Overview

The Rush Springs aquifer is composed of about 4,972 square miles (mi²) (3,182,080 acres) of Rush Springs aquifer bedrock deposits located in Blaine, Caddo, Dewey, Canadian, Comanche, Custer, Ellis, Grady, Kiowa, Major, Roger Mills, Stephens, Woodward, and Washita counties. The study area also includes the alluvium and terrace of the Canadian River and Washita River alluvial aquifers, as well as alluvium along the Little Washita River, Deer Creek, and a number of smaller tributaries of the Washita River that overlie the Rush Springs bedrock. Groundwater from this aquifer is used for irrigation, municipal, industrial, livestock, and self-supplied domestic and industrial water supplies in a number of local communities overlying the aquifer.

The 1973 Oklahoma Water Law (82 OK Stat § 82-1020.5) requires the Oklahoma Water Resources Board (OWRB) to conduct hydrologic investigations of the State’s aquifers (called groundwater basins) to support a determination of the maximum annual yield (MAY) over the life of the basin. The MAY is defined as the total amount of fresh groundwater that can be withdrawn annually from a groundwater basin while ensuring a minimum 20-year life of such basin. For bedrock aquifers, the groundwater basin life requirement is satisfied if, after 20 years of MAY withdrawals, 50 percent of the groundwater basin retains a saturated thickness of 15 ft. When a MAY has been established, the amount of land owned or leased by a permit applicant determines the annual volume of water allocated to that permit applicant. The annual volume of water allocated per acre of land is known as the equal proportionate share (EPS) pumping rate. To determine the MAY of a major groundwater basin, an investigation is conducted to obtain data and information related to the geology, hydrogeologic framework, and hydrogeology of an aquifer. A numerical groundwater-flow model can then be developed by using these data and information to conceptualize the flow system and evaluate effects of water use on the aquifer. The study described in this report to evaluate the Rush Springs aquifer is a cooperative effort between the U.S. Geological Survey (USGS) and the OWRB.

Objectives

The objectives of this project are to quantify the groundwater resources of the Rush Springs aquifer by developing a numerical groundwater-flow model, evaluate the effects of estimated equal proportionate share (EPS) on aquifer storage and streamflow for time periods of 20, 40, and 50 years into the future by using numerical groundwater-flow models, and evaluate the effects of present-day groundwater pumping over a 50-year period and sustained hypothetical drought conditions over a 10-year period on stream base flow and groundwater in storage by using a numerical groundwater-flow model.