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I I I I I I I I I I I I I I I I I I I ---- 0615 CONSUMPTIVE USE The change proposed for the state-owned portion of the Catlin water rights will include restrictions so that the rights of the other water users will not be injured. The amount that can be used for the permanent pool will be limited to actual consumptive use resulting from historic irrigation practice on the historic use land. Consumptive use is the amount of water that was consumed, both by evaporation from the fields and plant transpiration, during irrigation. Each of the ten individuals who sold Catlin shares was con- tacted to obtain the historic irrigation data shown in Table 4. A total of 2,336 acres was irrigated with the State's 2,097.58 shares. Acreages and crops grown with these shares . were estimated using the data from "Agricultural Statistics, 1971" used in the previous section and are shown below: Corn 32 .79 percent 766 acres Grain sorghums 12.62 percent 295 acres Alfalfa 43.62 percent 1,018 acres Sugar beets 6.58 percent 154 acres winter wheat 4.39 percent 103 acres crop irrigation requirements are the irrigation season con- sumptive use when sufficient irrigation water has been applied for optimum plant growth. They were computed for compact years 1949-73, using a modified Jensen-Haise method.l This method has been used successfully for irrigation scheduling in several areas in the western United States. The procedure involved calculation of monthly potential evapotranspiration using per- cent possible sunshine and mean air temperature. This theo- retical value was then multiplied by a coefficient,2 which allowed for the growth-stage of each crop. Monthl~ effective precipitation was determined using the USBR method and de- ducted from the plant evapotranspiration to form the monthly crop irrigation requirement. All climatological data were published records from the Rocky Ford weather station except percent possible sunshine data, Which were from the Pueblo weather station. Crop irrigation requirements averaged 4,533 acre-feet or 1.94 acre-feet per acre (Table 5) during this period. -15-