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The Colorado Irrigation Guide (U.S. Department of Agriculture Soil Conservation Service, <br />Denver, CO, revised December 1988) provides crop information at select locations throughout <br />the state. The locations within the SPDSS study area used in this analysis include Greeley and <br />Longmont. The available data is summarized in Table 4 for the crops with similar irrigation <br />water requirements (corn grain, corn silage, and sorghum). <br />Table 4 <br />Cro Information from the Colorado Irri ation Guide <br /> <br />Crop <br />Station <br />Growing Season <br />Growing Avg <br />ET <br /> Start End Days (inches) <br />Corn, Grain Longmont 5/1 9/20 142 21.66 <br />Corn, Silage Greeley 4/30 9/16 139 21.74 <br /> Longmont 5/5 9/10 128 19.74 <br />Grain, Sorghum Greeley 5/20 10/2 135 19.48 <br />As shown in Table 3, corn grain and corn silage have the same growing season beginning and <br />ending temperature criteria recommended for the Blaney-Criddle analyses. Table 4 shows that <br />they also have similar start and end of growing season dates at the Longmont climate station, <br />based on the Colorado Irrigation Guide. In addition, the crop coefficients, as shown in Figure 2, <br />are similar. Corn for silage is cut while the crop is still green, while corn for grain is allowed to <br />stay in the field until dry. However, the amount of additional water required by corn grain as it <br />is field drying is minimal. As shown in Table 4, at the Longmont climate station, the ET <br />increases by less than 2 inches. Therefore, it is recommended that corn grain and corn silage be <br />combined for both the irrigated acreage assessment and subsequent consumptive use modeling. <br />As shown in Table 3, corn grain and sorghum have the same ending temperature criteria <br />recommended for the Blaney-Criddle analyses. However, the beginning growing season <br />temperature varies by five degrees. Table 4 shows that both the beginning and ending of <br />growing season are later for sorghum than for corn grain at the Greeley climate station, based on <br />the Colorado Irrigation Guide. The dates are shifted by approximately 15 days; therefore, the <br />number of growing days is similar. The ET differs by only a little over 2 inches. Because the <br />growing season is different between corn silage and sorghum, it is recommended that sorghum <br />be considered a major crop type and not be combined into the corn category. If difficulties arise <br />in distinguishing between the spectral signature of sorghum and corn as the irrigated acreage <br />assessment progresses, this recommendation can be revisited based on the conclusion that the <br />irrigation water requirements are similar. <br />Small Grains and Winter Wheat <br />TR-21 does not distinguish between small spring grains. Oats, barley, and spring wheat are <br />grouped together and their growing season and crop coefficients, therefore their potential <br />consumptive use values, are the same. In addition, RTi has indicated that the spectral signature <br />of these crops types is very similar. Therefore, we recommend that these spring grains be <br />combined into a "small grain" category. This is consistent with the "small grain" crop group use <br />in RGDSS. <br />Page 8 of 13 <br />