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<br />. <br /> <br />u~lH5 <br /> <br />. <br /> <br />. <br /> <br />Wheeler, 1987a and 1987b, compared estimates of crop H using SCS-BC TR-21 <br />to crop H calculated by the Northern Colorado Water Conservancy District using a <br />calibrated Penman. They found that the ratio of SCS-BC TR-21 ET to calibrated Penman <br />ET for small grains, alfalfa, onions and cucumbers was 1.25, 1.14, 1.16 and 1.10, <br />respectively, (i.e. SCS-BC TR-21 underestimated H). Wheeler, 1987a and 1987b, also <br />found that the ratio of SCS-BC TR-21 ET to calibrated Penman H for sugar beets and <br />beans was 1.27 and 1.13, respectively (i.e. SCS-BC TR-21 overestimated). Similar <br />comparisons for feed com and silage corn found no difference in estimated values of ET. <br />Additionally, Wheeler, 1991, reported that near Greeley, Colorado, SCS-BC TR-21 over- <br />estimated the crop evapotranspiration of alfalfa, feed corn, silage corn, dry beans, sugar <br />beets and pasture grass by 36%,19%,32%,13% and 27%, respectively. Wheeler, 1991, <br />also found that SCS-BC TR-2l under-estimated crop evapotranspiration for onions and <br />small grains by 43% and 21 %, respectively. In Wheeler, 1991, SCS-BC TR-21 H values <br />were compared to ET values determined using Iysimeter based crop coefficients and Etr. <br />Not only does Wheeler's analyses show the need for local calibration, but also no single <br />correction factor can be applied for SCS-BC TR-21 to accurately estimate ET for all crops. <br />This is also demonstrated in the studies by Hill and Pochop and Burman. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />The reader probably noted that in 1987, Wheeler found that SCS-BC under- <br />estimated alfalfa ET, but in their 1991 finding, SCS-BC were over-estimated H. The 1987 <br />, study assumed a full water supply, i.e. the alfalfa crop was never stressed. The 1991 <br />study was a site specific study where the farmers generally only irrigated three times: in <br />the spring, after the first crop, and after the second crop. The 1991 farmers chose to <br />irrigate their more important cash crop (corn, beets and vegetables) later in the season <br />when the water supply was limited. <br /> <br />i. <br /> <br />. <br /> <br />Most if not all of the irrigated pasture grass in the Colorado River basin are fields <br />of native or improved native vegetation raised for hay. The irrigation is much more intense <br />than occurs on pastures. Additionally, in some areas a high groundwater table exists <br />beneath the field which results in the plant species being more of a water loving type <br />species (Siemer and Rumburg). Several studies have investigated the evapotranspiration <br /> <br />. <br /> <br />9 <br /> <br />. <br />