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Show PART III IRRIGATION POTEMIIAL Table 22 - Water supply available, Upper Colorado Region, in 1965 (Unit-1,000 acre-feet) Green Upper San Juan- Upper River Main Stem Colorado Colorado Subregion Subregion Subregion * Region Undepleted water supply (1914-65) 5,460 6,806 2,606 1^,872 Level of depletions (1965) 993 1,397 hlQ 2,8o8 Modified flow (1914-65) (excluding Main Stem evaporation) 4,467 5,409 2,188 12,064 Main Stem Reservoir evapo- ration normalized (1965) 67 576 643 Residual flow________________4,400______5,409______1,612______11,421 Large variations in annual discharge occur from year to year due to yearly variations in precipitation, and over periods of years due to long- term climatic trends. The average annual measured discharge of the Colo- rado River at Lee Ferry was 12,426,000 acre-feet for the 52-year period 1914-65, with extremes of 21,894,000 acre-feet in 1917 and 4,396,000 acre- feet in 1934. For the 17-year period 1914-30, the average discharge was 15,919,000 acre-feet per year, while for the 26-year dry cycle 1931-56, the average discharge was 11,183,000 acre-feet per year. These are resid- ual flows reflecting upstream depletions approximating 1,800,000 acre-feet in 1914 increasing to about 2,800,000 acre-feet in 1962. By 1965 the de- pletion was at the 3,451,000-acre-foot level, primarily due to the in- creased evaporation potential at the main stem reservoirs. As mentioned under "Water Rights" in Part II, the development of wa- ter in the Upper Colorado Region is governed by compacts, documents, and water laws, collectively referred to as "The Law of the River. " These documents and the restraints imposed are discussed in detail in Appen- dix III, Legal and Institutional Environments. The availability of water for development in the Upper Colorado Region is restricted by the Colo- rado River Compact which specifies that an aggregate of 75 million acre- feet over a 10-year period will be released for use in the Lower Colorado River Basin. Within the limits of institutional restraints, development of irri- gation wa-ter supplies is dependent, to a large extent, on development of regulatory storage. Current developments on tributary streams fully uti- lize available supplies during the late summer months, particularly dur- ing years of subnormal runoff. Availability of new irrigation water supplies is thus dependent on provision of seasonal storage regulation and regulation to smooth out the variation in annual flows. 62 |