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<br />o <br /> <br />,"') <br /> <br />N <br />~ Irrigation Consumptive Use <br />.... <br />~ In the Arizona portion of the Upper Colorado River Basin there are <br /> <br />26 Navajo Indian irrigation projects, 22 of which have been developed. <br /> <br />(In addition, there is a 40-acre non-Indian development at Lees Ferry.) <br /> <br />There are large year-to-year variations in the irrigated acreage and cropping <br /> <br />patterns due mainly to economic factors and the variability of the water supply. <br /> <br />Annual crop reports for the irrigation projects were obtained from <br /> <br />Bureau of Indian Affairs offices on the Navajo Reservation. The total <br /> <br />irrigated acreage by crop types (including estimates of miscellaneous <br /> <br />unreported irrigation and estimates of data missing from the BIA crop <br /> <br />reports) is shown in Table 4. <br /> <br />There are no records of reservoir storage, diversions, or return <br /> <br />flow that could be used to compute consumptive use on an inflow-outflow <br /> <br />basis. Consequently, it was necessary to estimate the historical irri- <br /> <br />gat ion consumptive use by using empirical consumptive use formulas with <br /> <br />historical climatic data as input. The potential irrigation consumptive <br /> <br />use was computed using the modified Blaney-Criddle formula. <br /> <br />The irrigation projects are subject to chronic water shortages. <br /> <br />Some of the project~ (notably Many Farms) have storage reservoirs and <br /> <br />thus receive a somewhat assured water supply. The remainder of the <br /> <br />projects are dependent on diversions from summer and fall thunderstorm <br /> <br />flood runoff. Thus, full potential consumptive use would be realized <br /> <br />only in occasional high water years. <br /> <br />After examining precipitation and runoff records, shortage adjust- <br /> <br />ment factors were selected by judgment. It was assumed that in water <br /> <br />years 1976, 1977, and 1978 the projects received only one-half of a full <br /> <br />potential supply. Since 1975 and 1979 were better hydrologic years, a factor <br /> <br />of two-thirds was applied to the theoretical results. <br />