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<br />u = <br />f = <br />. <br />t = <br />P = <br /> <br />. <br /> <br />0," 110 <br />:,~ 1. ~1; ~ <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />! <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />.. <br /> <br />. <br /> <br />USBR Procedure. The Methods Manual defines irrigation consumptive use as "the <br />net irrigation requirement (NIR) plus incidental losses." For each crop, the NIR is the crop <br />consumptive use (evapotranspiration or ET) minus effective precipitation. The method <br />used is the Soil Conservation Service's (SCS) modified Blaney-Criddle method (commonly <br />referred to as SCS-BC) as described in SCS, 1970 (aka TR-21). The SCS-BC equation is: <br /> <br />u <br /> <br />kc kt (f) <br /> <br />= <br /> <br />where, <br /> <br />monthly consumptive use, inches <br /> <br />(txp)(100 <br /> <br />mean monthly temperature, of <br /> <br />percentage of daylight hours of the year occurring during a <br />particular month <br /> <br />kc <br /> <br />monthly growth stage coefficient <br /> <br />= <br /> <br />kt <br /> <br />climatic coefficient calculated by <br />kt = .0173 t - .314. subject to kt > = 0.30 <br /> <br />= <br /> <br />NIR is multiplied by irrigated acreage to determine the irrigation consumptive use. This <br />procedure is performed for each crop in each hydrologic unit. The total irrigated agriculture <br />consumptive use for Colorado is determined by totaling the irrigation consumptive use <br />amount in the hydrologic units. <br /> <br />In the June 1991 report, the amount of irrigated area was determined using State <br />Agriculture statistics. The Methods Manual states "The greatest error in calculating <br />irrigation consumptive use is determining the irrigated acreage for the hydrology units." <br />This error is being corrected by a joint study of the USBR and the Colorado State <br />Engineer's Office; therefore, it will not be discussed in this investigation. <br /> <br />Several procedures determined for the 'Comprehensive Framework Study," which <br />was a report prepared in 1971. were utilized in the June 1991 report. The determination <br /> <br />4 <br />