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<br />The present condition (1977) well irrigated acreage in each quarter <br />township was developed from well registration data, information from <br />Nebraska Agricultural Statistics, and canal irrigated acreage data from <br />the Nebraska Department of Water Resources. The present well irrigated <br />acreage in the modeled area is estimated to be 1.4 million acres, of <br />which 936,000 acres are in the Platte River Basin (see table 9). <br /> <br />The total irrigabl e acreage is the same as that used in the PI atte <br />Level B Study, or approximately 4.3 million acres. Of this total, <br />773,000 acres are in the Blue River Basin. Deducting canal irrigated <br />acreage (453,000 acres) and lands which do not have a reliable ground- <br />water supply (586,000 acres) indicates that the future well irrigated <br />acreage could be as much as 3.2 million acres (table 9). The future <br />development was assumed to occur uniformly over a 50-year period. <br /> <br />Ground-water depletions due to well development were determined using a <br />hydro 1 ogi c mode 1 deve loped by the Bureau of Recl amat ion. The mode lis <br />of the monthly accounting type which determines evapotranspiration, <br />runoff, appl ied i rr igat i on water, net ground-water rech arge and the <br />change in soil moisture storage in the crop root zone as in function of <br />temperature, solar radiation and precipitation for each given crop-soil <br />combination. A total of 28 crop-soil combinations were considered in <br />conjunction with climatic data at 22 locations throughout the basin. <br /> <br />Using climatic data for the period 1941-1977, the average annual net <br />ground-water dep 1 et i on vari ed from 1 ess than 3 acre- inches per acre <br />irrigated in the far eastern portion of the basin to slightly more than <br />12 acre-inches per acre irrigated in the western portion of the basin. <br /> <br />The ground-water model was calibrated by simulating the historic <br />development for the period 1938-1977 and comparing the modeled ground- <br />water level changes with historic changes. <br /> <br />Each future condition was simul ated for a period of 50 years which was <br />imposed on the end of the historic development. <br /> <br />The flow model simul ates monthly flows at 16 locations in the study <br />area, which includes the North Platte River from below Guernsey <br />Reservoir and the South Pl atte River near Julesburg, Colorado, to the <br />Pl atte River near Duncan, Nebraska. The model is an accounting type <br />model developed from historic records and included diversion records of <br />canals. Where necessary, the historic irrigation diversions and section <br />gains during the 1940's and early 1950's were adjusted to reflect <br />present conditions. The model also included the current operating <br />criteria for the Tri-County and Sutherland systems and Lake McConaughy. <br />This criteria attempted to optimize hydroelectric generation after <br />meeting all irrigation demands with storage rights. Inflows were <br />provided by the Lower Missouri regional office for each condition <br />s imul ated . These i nc 1 uded fl ows for the South Pl atte at Jul esburg, <br />Larifllie River near Fort Laramie, and releases from Guernsey Reservoir. <br />Depletions due to well development were reflected by depleting the <br />section gains. <br /> <br />45 <br />