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<br />. <br /> <br />o"'e <br />I) 1\ llJ J \) <br /> <br />. <br /> <br />Reservoir Evaporation <br /> <br /> <br />Most reservoir evaporation in the San Juan River basin occurs at the <br /> <br /> <br />three reservoirs in the basin, excluding Navajo Reservoir. Evaporation was <br /> <br /> <br />estimated to average 2.8 acre-feet per surface acre per year. A monthly sum- <br /> <br /> <br />mary of estimated reservoir evaporation in the San Juan River basin is presented <br /> <br />in Appendix D. <br /> <br />. <br /> <br />. <br /> <br />Depletion Study - General <br /> <br /> <br />The irrigation water supply in the San Juan River basin is adequate in <br /> <br /> <br />most areas to meet the full crop water requirement. In several areas, most <br /> <br /> <br />notably on the smaller tributaries to the main stem, some water shortages occur, <br /> <br />especially in the later part of the ilTigation season during years of low stream- <br /> <br /> <br />flow. Depletion studies were performed to estimate the monthly effects on, <br /> <br /> <br />streamflow of crop ilTigation, evaporation, and reservoir operations. The studies <br /> <br /> <br />included separate analyses of depletion for river segments as listed in Table 4. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />A monthly summary of the total estimated stream depletions in the San <br /> <br /> <br />Juan River basin upstream from Navajo Reservoir is presented in Appendix E. <br /> <br />Since the historic annual streamflow at the Caraccas stream gaging station <br /> <br /> <br />has averaged 440,000 acre-feet per year, the average annual historic stream <br /> <br /> <br />depletions of 56,100 acre-feet per year amount to only about 13 percent of <br /> <br /> <br />the total annual streamflow. A description of the stream depletion analysis <br /> <br /> <br />for each river segment in the San Juan River basin is described in the following <br /> <br /> <br />sections. <br /> <br />. <br /> <br />. <br /> <br />Depletion Study for the San Juan River Basin Upstream from the Confluence <br /> <br /> <br />with the Navajo River <br /> <br /> <br />The irrigated land in the San Juan basin upstream from the Navajo River <br /> <br /> <br />totals about, 9,050 acres. The predominant crops are meadow grass with some <br /> <br /> <br />irrigated pastures. It is estimated that irrigation techniques generally used <br /> <br /> <br />in the basin result in a maximum ilTigation efficiency of about 50 percent. <br /> <br /> <br />Approximately 920 ilTigated acres are below 7000 feet in elevation, and about <br /> <br /> <br />8,130 acres are above 7000 foot elevation. For areas below 7000 feet in elevation, <br /> <br />. <br /> <br />-20- <br /> <br />. <br /> <br />, <br /> <br />j!."'<'~ <br />