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<br />Stud.ies withi n the Nebraska portion of the study area ut i1 fzed the <br />1941-1977 period and three digital computer models. The first was a <br />hydrologic model to determine ground-water recharge and net ground-water <br />pumpage for various crop-soil combinations throughout the basin. The <br />second was a ground-water model of the Pl atte River Basin in Nebraska <br />used to determi ne the effect of ground-water development on streamflow. <br />The third was a monthly flow model of the Platte River Basin from <br />Guernsey Reservoir on the North Platte River and Julesburg on the <br />South Pl atte River to Duncan. This third model integrated the results <br />of the ground-water model, upstream depletions, and existing operations <br />to determine the flows at Overton, Grand Isl and, and several other <br />locations. <br /> <br />The ground-water model was the finite difference model developed by the <br />Illinois State Water Survey. The area modeled included the Nebraska <br />portion of the contributing ground-water basin above Duncan, an area of <br />some 9 million acres, and a 1 million acre area of the Blue Basin <br />adjacent to the Platte River. <br /> <br />Model node spacing was every 3 miles, or equal to a quarter township. <br />The transmissivity data was the same as that developed for the Platte <br />Level B Study with the exception of those areas not modeled by the <br />Level B Study. Transmissivity in these areas was assumed to be <br />10,000 gallons per day per square foot. A ground-water storage coeffi- <br />cient of 30 percent was assumed for all locations. This value simulated <br />historic ground-water level changes. <br /> <br />The simulation of streamflow depletions due to ground-water development <br />is based on the following assumptions: <br /> <br />1. All geologic formations, from which pumping occurs, are hydraul- <br />ically connected to the streams modeled. These include the South <br />Platte, North Platte, and Platte Rivers, and Birdwood, Blue, Pumpkin, <br />and LOdgepole Creeks. <br /> <br />2. With the exception of the Blue River Basin, ground-water development <br />in adjacent basins will not affect flow of the Platte River, nor will <br />pumping in the Platte River Basin affect flows in adjacent basins. <br /> <br />3. The reduction in evapotranspiration from high water table areas was <br />simulated by assuming that the subirrigated area in any node was reduced <br />10 percent for each foot of ground-water level decl ine in that node. <br />The initial subirrigated lands, determined from pre-1940 soils classifi- <br />cation data, included 880,000 acres. The evapotranspiration rate for <br />the subirrigated land was assumed to be the same as for irrigated <br />grass. <br /> <br />4. The aquifer behaves as an artesian aquifer, i.e., the transmissivity <br />rema i ns constant regardl ess of the magn i tude of the ground-water 1 evel <br />decline. While this assumption is not correct in that ground water <br />occurs under water table conditions in much of the basin, the artesian <br />model requires considerably less data and computer storage. Thi~ <br />assumption results in overestimating streamflow depletions due to <br />ground-water development by no more than 10 percent. <br /> <br />42 <br />