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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Consumptive Uses and Losses <br /> <br />12 <br /> <br />.':) <br />::) <br />-:' <br />w <br />w <br />w <br /> <br />For this report, the modified Blaney-Criddle method was used for computing <br />irrigated crop consumptive use. This method, along with appropriate <br />crop-growth stage coefficients and the method for computing effective <br />precipitation, is described in Soil Conservation Service (SCS) Technical Release <br />No. 21. The values for consumptive use can vary significantly from year to year <br />because of the annual changes in climatic conditions. Additional information <br />required for applying this method includes mean monthly temperature and <br />precipitation values, planting dates and growing season data for each crop type, <br />and the total number of acres planted by crop at each site. With the exception <br />of Teec Nos Pos, Chinle Valley, Lukachukai, and Lees Ferry, no onsite <br />climatological data exist for the irrigation sites, and site values must be <br />generated from data collected at surrounding established weather stations. <br />Planting dates depend largely on the judgment of the local inhabitants and are, <br />therefore, not known. The number of acres planted by crop at each site varies <br />widely from year to year. Although several reports are available with <br />published values for irrigated acres, it is unclear if these values are meant to <br />represent potential irrigated acres, acres irrigated in the past, acres for which <br />farming permits have been issued, or actual acres currently under irrigation. <br /> <br />The crop acreage used to estimate consumptive use by irrigated crops for the <br />year 1990 is shown in table 4. These values were determined from a direct <br />field examination in 1985 by a party composed of representatives from <br />Reclamation, BIA, and the Navajo Nation. Since no acreage data were <br />available after 1985, the 1985 data were used to represent 1990. <br /> <br />Several factors complicated the modeling of irrigation consumptive use. <br />For example, the computer program used to model consumptive use assumes a <br />full water supply; if shortages exist, as they often do in thi.s semiarid study <br />area, adjustments must be made. These adjustments are complicated because <br />some of the projects have no storage water and may only have a reliable <br />irrigation water supply during spring runoff. Mter spring runoff, farming <br />becomes essentially dry land until rains in July and August provide <br />intermittent flows for irrigation. Finally, no ditch diversion records are <br />available to help estimate irrigation water supplies and shortages. <br /> <br />A full irrigation supply seldom exists at many of the projects. Generally, a full <br />supply is available for the Lees Ferry plot and the Many Farms, Round Rock, <br />Tsaile, Navajo Canyon, and Wheatfields Projects because of adequate stream <br />flows or because of available storage water. However, 1990 was a dry year and <br />the projects that do have a storage reservoir do not have carryover storage to <br />ensure against drought. For this report, Jerry Gonzales of the SCS estimated <br />the amount of crop water requirement met for 1990 for each crop area! <br /> <br />8 Data was obtained during a telephone conversation with Jerry Gonzales of the Fannington, <br />New Mexico SCS office, September 1991. <br />