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water, plant and soil surfaces so that vegetative growth and crop production are not <br />limited by lack of water. The consumptive irrigation requirement is the portion of the <br />consumptive water requirement that is not supplied by precipitation. The Blaney-Griddle <br />method (1970, U.S. Department of Agriculture) with an elevation adjustment (1984, <br />Pochop, et al) was used to estimate the consumptive irrigation requirements. <br />Temperature and precipitation data for Hayden were used in the analysis (1951-1986, <br />NOAA). Using the procedures and data described above, the consumptive irrigation <br />requirements for alfalfa averaged 21.69 inches for 1969 through 1986, and ranged from <br />18.65 inches in 1984 to 26.43 inches in 1977. The consumptive irrigation requirements <br />are summarized monthly in Table 8. <br />As previously noted, the consumptive irrigation requirement is an ideal <br />requirement; i.e., vegetative growth and crop production are not limited by water supply. <br />Many times, this theoretical requirement is not provided by the irrigation system. To <br />estimate the portion of the consumptive irrigation requirement satisfied by historical <br />diversions, herein called historical consumptive use, it is necessary to simulate the <br />operation of the irrigation system and determine if all or only a part of the consumptive <br />irrigation requirement was provided. In the simulation, a water budget procedure is used, <br />utilizing the consumptive irrigation requirements, the historical recorded diversions, the <br />maximum efficiency of the irrigation system, and the water-holding capacity of the crop <br />root zone. The water delivered monthly to the crop root zone that is surplus to the <br />consumptive irrigation requirement is stored in the root zone for use at times when the <br />delivery is insufficient to meet the consumptive irrigation requirement. Water delivered <br />to the root zone in any month that is surplus to the consumptive irrigation requirement <br />and the moisture-holding capacity of the crop root zone generally percolates downward <br />below the root zone and returns to the stream. The water budget procedure operates for <br />the entire growing season, accounting for all water delivered to the crop root zone. By <br />this procedure, the historical consumptive use may equal The consumptive irrigation <br />requirement under an irrigation ditch with a good water supply, appropriate maintenance, <br />and good management techniques. Also, the historical consumptive use may exceed the <br />effective diversion in a given month due to water stored in the root zone carried over <br />from the previous month. <br />Based on the soils information, slopes, distribution system and management, the <br />maximum irrigation efficiency was estimated to be 70 percent; meaning that 70 percent <br />of the historical delivery from J.C. Temple Reservoir No. I was available to meet the <br />consumptive irrigation requirement and to replenish soil moisture. Also, it was <br />10 <br />