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<br />0019~3 <br /> <br />The following sections discuss the various depletion categories and how data were developed to <br />support their estimates, Monthly irrigation depletions by unit for the 1970 to 1993 water years, are <br />shown in Appendix A. VII, Monthly non irrigation depletions for the 1970 to 1993 water years are <br />displayed by HUC in Colorado and by Unit in Arizona, New Mexico and Utah by depletion category <br />and are shown in Appendix B. <br /> <br />Historic Depletions <br /> <br />Irriaation Depletions <br /> <br />Historically, irrigation depletions in San Juan River Basin have occurred upstream of Shiprock, New <br />Mexico. Introduction of Navajo Indian Irrigation Project (NIIP) in 1976 resulted in a significantly <br />larger share of depletions occurring downstream of Farmington, NM, Operation of the Dolores <br />Project began in 1985, which includes expansion of the Montezuma Valley Irrigation Company <br />(MVIC) water supply, Dolores Project impacts to the San Juan River can largely be related to <br />increased return flows since the project's water supply is an import from Dolores River Basin. Most <br />Basin irrigation depletions occur in Colorado and New Mexico, so considerable time was spent in <br />deriving irrigated acreage, crop distribution, and crop consumptive use within these two states. <br />Arizona and Utah's crop consumptive use data was obtained from published sources, that being the <br />Colorado River System, Consumptive Uses and Losses Repons: 1971-75, 1976-80, and 1981-85. <br />Estimates of irrigation depletions within Utah for periods 1970. 1986-93 were calculated using the <br />best available data and the methods discussed in detail in later sections. The 1986-93 irrigation <br />depletions for the State of Arizona were taken from the Consumptive Uses and Losses. Arizona <br />Portion of the Upper Colorado River Basin Annual Repons: 1986, 1987, 1988. 1989. 1990. 1991. <br />1992, and 1993. Consumptive use data for 1970 were estimated utilizing climate data for the period <br />1970 but using the same crop distribution and irrigated area as what was used for 1971. <br /> <br />A GIS coverage developed for the Colorado River System Consumptive Uses and Losses Report <br />(CRSCU&L) for the states of Colorado and New Mexico was used as a source for irrigated acres, <br />Table 2-2 is a summary of Colorado data, Table 2-3 is a summary of New Mexico data as modified <br />by the New Mexico Interstate Stream Commission. How these data were used is discussed in detail <br />in later sections, <br /> <br />Consumptive Irrigation Requirement (CIR) Methodology <br />Irrigation depletions can be evaluated in one of two ways: <br />. diversion minus returns <br />. an empirical value based on local climate, irrigated area and water supply. <br /> <br />Since return flows were not measured, it was decided to estimate irrigation depletions based on a <br />empirically derived formula, the Soil Conservation Service Modified Blaney-Criddle method <br />(SCSMBC) as described in technical publication TR-21 (SCS, 1967). SCS has since been renamed to <br />Natural Resource Conservation Service (NRCS). Reclamation used computer program XCONS <br />which uses SCSMBC to compute irrigation water requirements, <br /> <br />DRAFT - SJRIP Hydrology Model Documentation <br />June 18, 1999 <br /> <br />10 <br />