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I I I I I I I I I I I I I I I I I I I Effective precipitation is the amount of precipitation falling during the growing season which supplies water for evapotranspiration, or in essence, reduces the irrigation requirements. It was calculated using Lamar precipitation data and a commonly used analysis method outlined in Technical Release No. 21 (U.S. Soil Conservation Service, 1970). This method incorporates monthly rates of potential evapotranspiration and monthly precipitation to estimate the amount of precipitation which becomes available for consumptive use after accounting for surface runoff and deep soil percolation losses. Precipitation data for Lamar used in the analysis are included in Appendix B and estimates of effective precipitation for each of the crops are included in Appendix C. Irrigation water requirements for each of the crops were calculated as potential evapotranspirlltion less effective precipitation and are included in Appendix C. The average annual crop irrigation water requirements in acre-feet per acre for the study period were 2.48 for alfalfa, 1.78 for grain corn, 1.51 for grain sorghum and 1.22 for winter grains. In addition to crop irrigation water requirements, estimates of potential evaporation from wet soils irrigated during the winter months of November to March were made. Soil evaporation takes place at rates dependent on climatic factors until the top few inches of soil dry out. Potential rates of soil evaporation were calculated using the Modified Blaney-Criddle Method and an assumed crop coefficient to represent wet soil conditions. Effective precipitation was determined and irrigation water requirements calculated using the same methods as for crops. Results of this analysis are included in Appendix C. This analysis resulted in an average annual winter irrigation water requirement of 0.08 acre-feet per acre or about one~inch of potential water loss. This estimate appears realistic when factors of low temperatures, intermittant snow cover, and frozen soils are considered. It also correpsonds to the approximate water holding capacity in the top three to six inches of soil, or the expected depth to which drying would occur during an average winter. Irrigation water requirement rates for the four crops and winter irrigation were combined with acreages and weighted to determine overall monthly and annual irrigation water requirements. Table IV includes these weighted irrigation water requirements in acre-feet per acre. The average annual irrigation water -9- ~ )('