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<br />, ~"~3 <br />Of)U0D <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />of the East, West, and Middle forks of the Mancos River. These ditches serve <br /> <br /> <br />about 7,600 acres of the more intensely irrigated and productive lands in the <br /> <br /> <br />Mancos basin. For the 1951 through 1980 study period, the average annual direct <br /> <br /> <br />flow diversions for these ditches was about 19,500 acre-feet while the average <br /> <br /> <br />annual diversion of storage water released from Jackson Gulch Reservoir was <br /> <br /> <br />about 4,000 acre-feet. The diversions for these 22 ditches represents about <br /> <br /> <br />67 percent of the total direct flow diversions and about 70 percent of the Jackson <br /> <br /> <br />Gulch Reservoir storage water releases. <br /> <br />The results of the operation study indicate that on the average, the poten- <br /> <br />tial irrigation consumptive use requirement could be satisfied with direct irriga- <br /> <br /> <br />tion water, storage releases, and soil moisture carryover. This results in an <br /> <br />average annual irrigation depletion (for the 2Z ditches) of about 16,100 acre-feet. <br /> <br />The remaining water rights irrigate the acreage lying north of town of <br /> <br /> <br />Mancos which is irrigated on a more infrequent basis, primarily to improve <br /> <br /> <br />the range pasture. The irrigation depletion for these areas was estimated using <br /> <br /> <br />the average annual direct flow diversions (about 1,600 ac-ft/yr) and Jackson <br /> <br /> <br />Gulch storage releases (about 1,800 ac-ft/yr) for the water rights which serve <br /> <br /> <br />these lands. Assuming an overall irrigation efficiency of 60 percent, the average <br /> <br /> <br />annual depletion on these lands 'was estimated to be about 2,000 acre-feet per <br /> <br /> <br />year. Based upon the operation study of the historic irrigation practices, it <br /> <br /> <br />is estimated that the average annual irrigation depletion for the entire Mancos <br /> <br /> <br />basin for the 1951 through 1980 period was 18,100 acre-feet per year. <br /> <br />Jackson Gulch Reservoir has a water surface area of about 225 acres, <br /> <br /> <br />and the average annual net evaporation was estimated to be about 700 acre-feet <br /> <br /> <br />per year. Since this loss was not included in the irrigation depletion analysis <br /> <br /> <br />described above, the total depletion to the Mancos River associated with reservoir <br /> <br /> <br />storage and irrigation was estimated to average 18,800 acre-feet per year. <br /> <br />A more detailed analysis of irrigation depletion was made for the spring <br />runoff period of February through June for use in monthly stream gain calcula- <br /> <br />-17- <br /> <br />ct <br />