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<br />" .OG1J~O <br />Task 1.14(b) - Consumptive Use Prototype Development <br /> <br />generally dry, most of total precipitatioli can be available for consumptive use. But, <br />irrigation can change this assumption by increasing the moisture in the root zone and <br />thereby decreasing the effective rainfall. Precipitation from a storm immediately after <br />the application of irrigation water will not be absorbed into water saturated soil and <br />will result in high amounts of run off and consequently low amounts of effective <br />rainfall. <br /> <br />The consumptive use rate of the crops can change the amount of effective <br />precipitation. If the consumptive use rate is high the moisture in the soil will be <br />depleted rapidly and more precipitation can be expected to be absorbed into the <br />moisture-depleted soil. Conversely, if the consumptive use rate is low more water will <br />be left in the soil and consequently more water will be lost to run off and deep <br />percolation. <br />Monthly and seasonal effective rainfall can be expected to vary widely from year to <br />year. Other factors being equal, effective rainfall can be expected to vary from year to <br />year in direct proportion to the variance in total rainfall. <br /> <br />2.1.3 Estimating Consumptive Use Using the Blaney-Criddle Equations <br />Consumptive use can be estimated from climatic data when the size of an area being <br />considered, such as Colorado's Western Slope, makes it infeasible to do on-site <br />calculations of consumptive use. For this purpose one of the methods selected for <br />CRDSS is the Blaney-Criddle method with modifications adopted by the Soil <br />Conservation Service (SCS). This method is commonly referred to as SCS Blaney- <br />Criddle. The development of the Blaney-Criddle method is presented in Jensen (1990). <br />The earlier research was published in Blaney et al. (1952), and later modified by <br />USDA (1970). <br /> <br />The SCS Blaney-Criddle procedure for estimating evapotranspiration is well known <br />in the western United States. The original relationship was developed and intended for <br />seasonal estimates and the basic assumption is that evapotranspiration varies directly <br />with the sum of the products of mean monthly airtemperature and monthly percentage <br />of annual daytime hours for an actively growing crop with adequate soil moisture. The <br />original relationship was developed and intended for seasonal estimates. This can be <br />expressed mathematically as: <br /> <br />u = kf <br /> <br />U=KF=2.kf <br />U = estimated evapotranspiration in inches for the growing season <br />K = Empirical consumptive use coefficient <br />F = The sum of Monthly consumptive use factors <br />k = The monthly consumptive use coefficient <br />f = tp/lOO, where t = mean monthly air temperature and p = the mean monthly <br />percentage of annual daytime hours. <br />u = Monthly consumptive use of the crop in inches or millimeters. <br />The followinl!: modifications to Blanev-Criddle were made bv USDA (1970): <br /> <br />7/13/94 - Draft of Task Memorandum <br /> <br />4 <br /> <br />IDS Group, CSU <br />