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<br />-, G'~ . <br />U . '/1 <br /> <br />Table 4 summarizes the recharge and conveyance facility requirements for <br />each of the alternative plans evaluated. <br /> <br />Water Supply <br /> <br />Historical Flows <br /> <br />The source of surface water available for ground-water recharge comes <br />from the South Platte River, During the period 1947 through 1974, <br />the average annual flow of the South Platte River at the Narrows <br />Dam site was 422,700 acre-feet, and the average annual flow at Balzac, <br />Colorado, was 318,100 acre-feet. Using the South Platte River point <br />flow study data (USBR, 1967), it was estimated that an average of <br />237,000 acre-feet (1947-1974) of the South Platte River flow at the <br />Narrows Dam site was not utilized annually. This ranged from 4,200 <br />acre-feet in 1964 to 994,800 acre-feet in 1973. <br /> <br />Flows Divertibl e for Recharge <br /> <br />Based on a hydrograph analysis of historical flows and the storable <br />flows for Narrows Reservoir, the average annual supply directly <br />divertible from the South Platte River for recharge was estimated to <br />be 100,300 acre-feet. The annual divertible flows were estimated to <br />range from 3,400 acre-feet in 1954 to 307,400 acre-feet in 1973, <br />These values were calculated using a diversion season which extends <br />from March through3November, a total canal capacity for the recharge <br />system of 1,370 ft Is, and the operational constraints imposed by the <br />recharge facilities. Due to the need for alternate wetting and drying <br />periods to maintain infiltration rates in the recharge ponds, the <br />operational constraint consists of recharging a maximum of 81,800 <br />acre-feet in a 60-day period. <br /> <br />" <br /> <br />The annual river diversion requirement for the entirely new conveyance <br />system was used in the operation study and is 6 percent less than for <br />the combination system. The flows divertible for recharge used in <br />the operation study were smaller than they would have been if they <br />were calculated based on the canal capacity of the combined system. <br /> <br />Ground-Water Operation Study <br /> <br />A ground-water operation study was used to determine the yield which <br />could be expected from the ground-water storage of surplus flows and <br />from the subsequent pumping of this water to meet the computed supple- <br /> <br />44 <br />