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temperatures reported in TR-21 for both the ASCE Standardized and the modified Blaney- <br />Criddle methods. Because the source of the climate data was the same for both the ASCE <br />Standardized and Modified Blaney-Criddle runs, growing season adjustment was not necessary <br />under this task and only adjustment of the modified Blaney-Criddle crop coefficients was <br />required in developing calibrated coefficients. <br />Calibrated coefficients for alfalfa, corn grain, dry beans, grass pasture, small grains and sugar <br />beets were developed at the Fort Collins, Fort Lupton, and Greeley climate stations for the <br />available period of record. Because PCU is not a linear function of the crop coefficients, <br />developing the coefficients was an iterative process. The following steps describe the process <br />used to develop calibrated coefficients for each crop type at each of the four climate stations: <br />1. The ASCE Standardized option in StateCU was run with daily climate data. <br />2. The modified Blaney-Criddle option in StateCU was run with monthly climate data <br />using the TR-21 crop coefficients. <br />3. The average monthly ASCE Standardized PCU estimates (Step 1) were divided by <br />the average monthly modified Blaney-Criddle PCU estimates (Step 2). <br />4. The resulting monthly factors (Step 3) were applied to the modified Blaney-Criddle <br />crop coefficients in deriving calibrated crop coefficients. For crop coefficients based <br />on day of year (perennial crops including alfalfa and pasture grass), the monthly <br />factors were applied to the monthly TR-21 crop coefficients. For crop coefficients <br />based on percentage of growing season (annual crops including corn grain, dry beans, <br />small grains, and sugar beets), the monthly factors were applied to the crop <br />coefficients during the average growing season for each crop type at each climate <br />station. <br />5. The modified Blaney-Criddle option in StateCU was run using the calibrated crop <br />coefficients. <br />6. The monthly ASCE Standardized PCU estimates (Step 1) were divided by the <br />monthly modified Blaney-Criddle PCU estimates based on the calibrated crop <br />coefficients (Step 5). <br />7. Steps 4 through 6 were repeated until the monthly factors calculated in Step 6 were <br />1.0 for each month, generally taking around three to four iterations. <br />As compared to the Sterling climate station, the Fort Collins, Fort Lupton, and Greeley stations <br />are located relatively close to each other and have similar elevations. The modified Blaney- <br />Criddle method was run using the calibrated crop coefficients for the Fort Collins, Fort Lupton, <br />Greeley, and Sterling climate stations at the Greeley station. An example is shown in Figure 5 <br />for alfalfa. A comparison showed that the seasonal PCU using the Fort Collins, Fort Lupton, and <br />Greeley calibrated crop coefficients was typically within a few percentages of each other while <br />the seasonal PCU using the Sterling coefficients was on the order of ten percent different. <br />Therefore, the Fort Collins, Fort Lupton, and Greeley coefficients were averaged and will be <br />used to represent the Upper Plains (Water District 1, 2, and the lower portions of Water Districts <br />3, 4, 5, 6, 7, 8, and 9). The Sterling calibrated coefficients will be used to represent the Lower <br />Plains (Water District 64). <br />Task59_l.doc (updated January 11, 2008) 12 of 32 <br />