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CRDSS <br />TASK MEMORANDUM 1.14-7 <br />Consumptive Use Model <br />Soil Conservation Service Blaney-Criddle as Implemented in XCONS2 <br />1.0 ISSUE <br />This task memorandum presents the Soil Conservation Service (SCS) Blaney-Criddle equation <br />implemented in XCONS2 for estimating evapotranspiration (ET) for irrigated agricultural areas. <br />2.0 DISCUSSION/ANALYSIS <br />The SCS Blaney-Criddle Equation for Estimating ET <br />Introduction <br />The form of the Blaney-Criddle equation used in XCONS2 is the original Blaney-Criddle equation with <br />modifications adopted by the SCS. This method is commonly referred to as SCS Blaney-Criddle and is <br />described in the U.S. Department of Agriculture Soil Conservation Service?s Irrigation Water <br />Requirement, Technical Release No. 21, revised September 1970 (TR-21) (USDA 1970). Some <br />additional modifications for XCONS2 were implemented, and this methodology will be referred to as <br />SCS Blaney-Criddle (XCONS2). This method has been used by the U.S. Bureau of Reclamation <br />(USBR) to prepare the Colorado River System Consumptive Uses and Losses Report, 1981-1985. <br />Background <br />The original Blaney-Criddle research was published in Blaney et al. (1952), and was later modified as <br />TR-21. The development of the SCS Blaney-Criddle method is presented in Jensen et al. (1990). The <br />basic relationship used for the SCS Blaney-Criddle was developed for seasonal estimates. This basic <br />relationship assumes that ET varies directly with the sum of the products of mean monthly air <br />temperature and monthly percentage of annual daylight hours for an actively growing crop with adequate <br />soil moisture. The implementation of this method will now include the soil moisture characteristics and <br />possible water supply limitations. This is expressed mathematically as: <br /> u = kf . . . . . . . . . . . . . . . . . . . . . . (1) <br /> U = KF = the sum of all kf . . . . . (2) <br />where U is the estimated ET in inches for the growing season; K is the empirical consumptive use <br />coefficient; F is the sum of monthly consumptive use factors; k is the monthly consumptive use <br />coefficient; u is the monthly consumptive use of the crop in inches or millimeters; and f = tp/100, where t <br />is the mean monthly air temperature and p is the mean monthly percentage of annual daytime hours. <br />The stage of crop growth is a primary variable that must be recognized since plants use more water <br />during a rapid growth phase. Variations in the growing season from year to year are expected to be <br />larger with annual crops than with perennial crops, such as alfalfa or grass. In addition to variations in <br />1 <br />A275 01.09.95 1.14-7 CSU IDS <br />