Ground-water use for irrigation from the Ogallala Formation }'as in-creased rapidly in the Upper Republican Natural Resources District in southwest Nebraska. Water levels declined as much as 16 feet between 1952 and 1975. Discharge of the aquifer to streams was reduced by as much as 19 percent between 1967 and 1975. Quantification of the hydrogeologic system was provided by the development, testing and use of simulation models describing the soil zone and ground-water/surface-water system. Models were linked through source-sink terms. The ground-water/surface-water model was used.to predict changes in water levels and streamflow caused by unrestricted irrigation-well installation and by no new wells after 1975. Water levels may decline as much as 140 feet in two areas by 2000 if installation of new wells is unrestricted. By 2000, water-level declines o-er the remainder of the area would be less than 60 feet under continued development and less than 40 feet with no new wells after 1975. The base flow of Frenchman, Stinking Water, and Spring Creeks coulA be reduced by more than.90 percent with no new well and eliminated by 1992 under continued development.
