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The hydrogeologic system in south-central Nebraska, which has been altered by surfaceand ground-water irrigation, has been described and modeled to evaluate quantitatively the effects of management practices,on water levels, streamflow, and surface-water seepage. Since the early 1940 f s when the Central Nebraska Public Power and Irrigation District began operation south of the Platte River in soutlv-central Nebraska, seepage from this surface-water irrigation system has resulted in ground-water level rises of as much as 110 feet above the water levels in 1940 in the northwestern part of the study area. Ground-water irrigation has increased substantially throughout the study area since the 1940's; nevertheless, only minor water-level declines have occurred over much of the study area. The depth to water ranged from less than 1 foot in several locations to 350 feet in the northwest corner of the study area for the 1940 to 1981 time period. The saturated thickness of the aquifer decreased from west to east and from north to south and ranged from near zero along the southern boundary of the study area to greater than 600 feet in the northwest corner for the 1940 to 1981 time period. Transmissivity and specific yield of the aquifer indicated little change during the 1940 to 1981 time period, even though the saturated thickness of the aquifer increased in the northern part of the study area. Transmissivity ranged from 100 to 20,000 square feet per day in 1940, and from 100 to 25,000 square feet per day in 1981. Specific yield ranged from 0.08 to 0.26 for the entire time period. The hydrogeologic system was subdivided into four components: surfacewater system, soil zone, unsaturated zone, and saturated zone. Computer programs were developed or obtained to represent the hydrologic regime in each component except the unsaturated zone. A two-dimensional, finite-element, ground-water flow model (RAQSIM) was developed to represent the hydrogeologic systems in this 5,600 square-mile study area. The model was calibrated for the time period 1940 through the spring of 1981. A comparison of the computed and measured 1981 water levels was favorable. In most of the study area, the differences between computed and measured water levels were less than 10 feet. In the southern part of the study area, water-level differences of 15 to 20 feet occurred. An examination of hydrographs from eight observation wells showed similar trends between the observed and computed water levels during the calibration period. The ground-water flow model was used to simulate a management alternative that consisted of no additional irrigation development beyond the 1981 level. Projected water levels indicated maximum rises above the 1981 computed water levels in the northwestern part of the study area of 40 feet by the year 2000 in a 50 square-mile area and 60 feet by the year 2020 in a 12 square-mile area.

Hydrogeology of the tri-basin and parts of the lower Republican and central Platte natural resources districts, Nebraska