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<br />. <br /> <br />. <br /> <br />. <br /> <br />I,'\r,'~'3c? <br />UtJI 0' ,) '.,j <br /> <br />this station appear to be abnormally high on a plot of weather station elevation <br /> <br /> <br />vs. average monthly temperature, whereas the average of the Fort Lewis and <br /> <br /> <br />Durango temperatures matches closely with the elevation of the Mancos valley. <br /> <br /> <br />The average mean monthly temperatures adopted for the study period are as <br /> <br />follows: <br /> <br />APR <br />42.5 <br /> <br />MAY <br />51.1 <br /> <br />JUN <br />59.8 <br /> <br />JUL <br />66.4 <br /> <br />AUG <br />64.2 <br /> <br />SEP <br />57.1 <br /> <br />OCT <br />47.5 <br /> <br />Precipitation data for the consumptive use calculation were obtained <br />from the weather station at the Town of Mancos (which does not record tempera- <br />tures). For the 1951 through 1980 study period, the average April through October <br />precipitation at Mancos was 9.5 inches, of which 7.1 inches are considered <br />effective in meeting a portion of the irrigation requirement. <br /> <br />Using these data, the potential consumptive use of irrigation water was <br /> <br /> <br />estimated to be about 2.13 acre-feet per acre, provided that the irrigation water <br /> <br /> <br />supply is sufficient in time and quantity. <br /> <br />Operation Study <br /> <br /> <br />In general, the irrigation water supply in the Mancos basin is inadequate <br /> <br /> <br />to satisfy the crop consumptive use requirement, unless supplemented with <br /> <br /> <br />reservoir releases from Jackson Gulch Reservoir. Therefore, an operation study <br /> <br /> <br />was performed in which the average monthly historical ditch diversions, including <br /> <br /> <br />reservoir releases, were compared to the average monthly crop irrigation require- <br /> <br /> <br />ments to determine what portion of the irrigation water was actually consumed. <br /> <br /> <br />In the study, an allowance was made for the accumulation of moisture in the <br /> <br /> <br />soil profile from surplus irrigation and return flows, which was available for <br /> <br /> <br />subsequent crop consumption. <br /> <br />The operation study was performed using the irrigated acreages and <br /> <br /> <br />diversions for 22 major ditch systems located downstream of the confluence <br /> <br />-16- <br />