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I I I I I I I I I I I I I I I I I I I increased from over 288,000 acre-feet during the 1928 through 1952 water year period to almost 337,000 acre-feet during the 1953 through 1979 water year period (Table 8). Ground Water The valley-fill alluvial aquifer of the South Platte River basin is estimated to store over 14 million acre-feet of ground water (Ted Hurr, personal communication, U.S. Geological Survey, JUly 1982). An estimated 1.2 million acre-feet of alluvial ground water is withdrawn annually through pumping pricipally for irrigated agriculture (Toups Corporation, 1975). The width of the alluvium within the South Platte River channel ranges from 3 to 8 miles (Figure 8), and depths to water in the alluvium range from 10 to 100 feet. The estimated saturated thickness of the aquifer ranges from zero to 240 feet. The transmissivity of the aquifer ranges from 100,000 to as large as 1,200,000 gallons per day per foot (gal/d/ft) in localized areas (Hurr and others, 1975). The alluvium is recharged by precipitation, applied irrigation water, and leakage from canals and reservoirs. Recharge from applied irrigation water and leakage from canals to the alluvium is estimated to be 45 to 50 percent of the diverted irrigation water (Hurr and others, 1975). Ground water withdrawals lower the water table of the alluvial aquifer and loca11y may cause the ground water gradient adjacent to the stream to reverse. Consequently, at certain times of the year and particularly durin~ significant pumpinq of al]llvial wells, the direction of flow may 90 from the river to the adjacp.nt alluvium (Hurr and others, 1975). From 1947 to 1970, measured water level declines in the alluvium of the mainstem South Platte pjver have been as qrpat as 23 feet. This represented a decrease in ground water storage of 456,000 acre-feet, or approximately 19,000 acre-feet per year.. Tnp. neclines, whicn were nighly variable both locally and with time, apparpntly were the result of the -33-