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<br />. <br /> <br />OOi{44 <br /> <br />. <br /> <br />. <br /> <br />By using COAGMET data, the USBR can accurately compute the reference crop ET <br />at the six Colorado River basin sites using either Penman Monteith or 1982 Kimberly <br />Penman; then monthly calibration coefficients could be determined by comparing Eto <br />calculated using a temperature equation to Eto or Etr. The monthly calibration factor could <br />then be used at the temperature-only stations in the Colorado River basin. As more data <br />become available from COAGMET, the calibration coefficients could be updated. <br /> <br />. <br /> <br />COAGMET does not have a weather station in Grand or Routt County. Estimates <br />of Eto and Etr using the procedure suggested above would be more accurate if a station <br />were installed in either or both counties. It is recommended that the USBR contact Dr. <br />Duke about the possibility of installing an additional station or stations. <br /> <br />.. <br /> <br />. <br /> <br />Calibrate SCS-BC. Jensen et al. recommended local or regional verification or <br />calibration is advisable for any method used to estimate crop evapotranspiration. This is <br />particularly advisable for the SCS-BC because the monthly coefficients (k) contain a <br />meteorological component and d<lY length (i.e. percent of daylight hours [pH does not <br />adequately reflect solar radiation's impact on crop ET (Jensen et al. and Pochop and <br />Burman). <br /> <br />r <br /> <br />. <br /> <br />. <br /> <br />The need for location calibration is documented in several studies including Hill <br />et al., 1989: Pochop and Burman; Kruse; Hill et aI., 1983; and Walter et aI., 1991. Hill <br />et aI., 1989, reported seasonal ET estimates using SCS-BC TR-21 coefficients underesti- <br />mated calibrated ET by 34% and 12 % for irrigated meadow and alfalfa, respectively. <br />Pochop and Burman reported that SCS-BC TR-21 estimates of seasonal ET for alfalfa and <br />mountain meadows were 56% and 49%, respectively, of measured ET. Hill et al., 1983, <br />found that the ratio of attainable ET to calculated SCS-BC TR-21 ET for corn and alfalfa <br />was 1.49 and 1.02, respectively at Grand Junction. These studies were conducted in an <br />area where the geography and climate is similar to the areas within Colorado's Colorado <br />River basin; therefore, they demonstrate the need for local calibration in Colorado. <br /> <br />( <br />- \ ~. <br />'f,\"~~ C <br /> <br />. <br /> <br />. <br /> <br />8 <br /> <br />. <br />