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40 The information shown on Table 4 presents an analysis of basin water use with various developments. An examination of the historic water supply reveals that the measured surface flows contributed to the main river valley area have averaged 530',000 acre-feet for the 6-year period 1952-l957. This quantity includes the measured outflow from the main tributary streams; Clear Creek, Big Thompson River, St. Vrain River, and the Cache la Poudre River, as well as the measured flow of tfle main stem South Platte River at Denver and metropolitan area sewage return flows. The measUred surface flows are augmented by an unmeasured surface and subsurface , supply estimated to average 200,000 acre-feet per year. The values on line 3 represent the total water supply available to the main ri ver area. They are the total of values shown on lines 1 and 2. The increased gravity supplies shown under Study E and F conditions reflects evaporation savings from abandoned off-channel reservoirs plus elimination of consumptive uses by lands inundated in the potential reservoir areas. In Study G conditions (1990 without project) the total gravity supply is increased by 190,000 acre- feet per year (720-530). Of this increase 160,000 acre-feet is attributed to increased sewage return flow and 30,000 acre-feet to full operation of the Colorado-Big Thompson Project. The results of our ground water studies for the South Platte Basin are shown on line 4. The value on the top of the slash is the calculated average quantity of water that was actually pumped. The value underneath the slash is the calculated quantity of water that would have to be diverted from the river to provide an equivalent supply at the farm headgate or at the pump site. For example, under Study A conditions, it would be necessary to divert 520,000 acre-feet of water at the river to provide the same supply to the crops as might be obtained by pumping 300,000 acre-feet. Under historic conditkns the equivalent pump supply is nearly equal to the measured gravity supply. Under 1990 conditions the pump supply is about two-thirds of the gravity supply since the gravity supply increases and the pumping level diminishes. These values clearly indicate the importance of ground water pumping for irrigation in the main stem South Platte River Valley Basin. The values shown on lines 5 through 9 summarize the results of the water utilization studies made for each of the plans studied., There is little difference in remaining river diversion shortages (line 8) between comparable plans. For example, under Study J conditions the remaining shortage with Weld County Reservoir would be 147,000 acre-feet per year while the comparable shortage with Narrows Reservoir (Study K) would be 156,000 acre-feet per year. Either reservoir provides a facility for greatly enhancing the use of the basin water supply (the calculated shortage under without project conditions is 310,000 acre-feet).