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<br /> <br />Ground-water withdrawal estimates made by this study for the High Plains <br />region are about 10 percent less than estimates made for 1980 as part of the <br />High Plains Regional Aquifer Study (Heimes and Luckey, 1983, p. 34). <br />Irrigation in the High Plains has declined over this period because of <br />economic reasons. Ground-water irrigation withdrawal estimates are difficult <br />to make for the South Platte and Upper Arkansas subregions because of complex <br />relations between surface-water and ground-water availability and use. Ground <br />water is used primarily in these areas as supplemental irrigation water when <br />surface water is scarce. A complicating factor in estimating ground-water <br />withdrawals is the absence of much real data on ground-water pumpage. A 1970 <br />estimate of irrigation withdrawals within the South Platte River basin (U.S. <br />Army Corps of Engineers, 1977) was 40 percent larger than the estimate made by <br />this study for the South Platte subregion. However, estimates of irrigated <br />acreage also were 40 percent larger for that study. The 1970 study estimated <br />that ground water provided 37 percent of irrigation water while this study <br />estimated the proportion to be 30 percent. <br /> <br />Irrigation water use (12,400 million gallons per day) accounts for 60 <br />percent of all water use in Colorado. Irrigation in Mesa County and Weld <br />County each used about 10 percent (1,300 million gallons per day) of the total <br />irrigation withdrawals. Each of the major river basins (the Colorado, Platte, <br />Arkansas, and Rio Grande) contains a leading irrigation-water-use county <br />(fig. 8). It is estimated that 37 percent of total irrigation withdrawals <br />are consumptive Use. Conveyance losses (23 percent of withdrawals) include <br />some consumptive use (water lost through evaporation or used by phreatophytes) <br />and some return flows (unused water, seepage to the ground-water table). <br />Tabular summaries of irrigation water-use data are listed in table 8 (for <br />counties) and table 19 (for hydrologic subregions) in the "Supplemental Data" <br />section at the back of this report. <br /> <br /> <br />EXPLANATION <br /> <br />WATER USE. IN <br />MilliON GAllONS <br />PER DAY <br /> <br />D leu than 26 <br /> <br />t: ......125 to 100 <br /> <br />Iw';.ll00 to 200 <br /> <br />200 to 400 <br /> <br />III Greeter than 400 <br /> <br />Figure 8.--Irrigation water use, by counties, during 1985. <br /> <br />20 <br />