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Studies witnin the Nebraska partion of the study area utilized the 1941-19-77 period and three digital computer models. The first was a hydro 1 ogi c model to determi ne ground-water recharge and net ground-water pumpage for various crop-soil combinations throughout. the basin. The second was a ground-water model of the PI atte River Basin in Nebraska used to determine the effect of ground-water development on streamflow. The third was a monthly flow model of the Platte River Basin from Guernsey Reservoir on the North Platte River and Julesburg on the South Platte River to Duncan. This third model integrated the results of the ground-water model, upstream depletions, and existing operations to determine the flows at Overton, Grand Island, and several other locations. The ground-water model was the finite difference model developed by the III inois State Water Survey. The area modeled included the Nebraska portion of the contributing ground-water basin above Duncan, an area of some 9 million acres, and a 1 million acre area of the Blue Basin adjacent to the Platte River. Model node spacing was every 3 miles, or equal to a quarter township. The transmissivity data was the same as that developed for the Platte Leve I B Study with the except i on of those areas not mode 1 ed by the Level B Study. Transmissivity in these areas was assumed to be 10,000 gallons per day per square foot. A ground-water storage coeffi- cient of 30 percent was assumed fOr all locations. This value simul ated historic ground-water level changes. The simulation of streamflow depletions due to ground-water development is based on the following assumptions: 1. All geologic formations, from which pumping occurs, are hydraul- ically connected to the streams modeled. These include the South Platte, North Platte, and Platte Rivers, and Birdwood, Blue, Pumpkin, and Lodgepole Creeks. 2. With the exception of the Blue River Basin, ground-water development in adjacent basins will not affect flow of the Platte River, nor will pumping in the Platte River Basin affect flows in adjacent basins. 3. The reduction in evapotranspiration from high water table areas was simulated by assuming that the subirrigated area in any node was reduced 10 percent for each foot of ground-water level decl ine in that node. The initial sub irrigated lands, determined from pre-1940 soils classifi- cation data, included 880,000 acres. The evapotranspiration rate for the subirrigated land was assumed to be the same as for irrigated gr ass. 4. The aquifer behaves as an artesian aquifer, i.e., the transmissivity remains constant regardless of the magnitude of the ground-water level decline. While this assumption is not correct in that ground water occurs under water table conditions in much of the basin, the artesian model requires considerably less data and computer storage. This assumption results in overestimating streamflow depletions due to ground-water development by no more than 10 percent. 42