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if crop coefficients are not defined for April, PCU cannot be estimated. Further analysis <br />show that excluding PCU in April has a minimal effect on average annual PCU both at <br />the ditch level and basin-wide. Therefore, further review of the original lysimeter data <br />used in the Walter Study to determine an appropriate April crop coefficient is not <br />recommended. <br />Elevation Adjustments -The high-altitude crop coefficients were developed based on <br />lysimeter sites located between 8,900 feet and 9,200 feet above mean sea level. There is <br />concern that it may not be appropriate to adopt the coefficients, without adjustment, at <br />lower elevations (down to 6,500 feet). We agree that the use of these coefficients may <br />overestimate potential ET at lower elevations. <br />We first investigated this issue by looking at other quality-controlled lysimeter studies at <br />lower elevations. The Gunnison Study discussed in the Task 59.1 memorandum included <br />coefficients developed for a range of elevations (7,500 feet to 10,500 feet) that are very <br />similar to the recommend high-altitude coefficients. <br />There are no CoAgMet or climate stations with the daily data required for Penman <br />Monteith at higher elevations. However, three CoAgMet stations (Dove Creek DVCO1, <br />Yellow Jacket Y7K01, and Center CTRO1) are between 6,600 feet and 7,700 feet and <br />have at least 5 years worth of the daily data required for the Penman Monteith <br />calculation. We compared the estimates of ET using Penman Monteith to the estimates <br />of ET using Blaney-Criddle with the high-altitude coefficients at the three locations. The <br />results were non-conclusive. Penman Monteith estimated ET at Dove Creek was higher <br />than Blaney-Criddle high-altitude estimates by more than 10 percent for every analysis <br />year, whereas Penman Monteith estimated ET at both Yellow Jacket and Center was <br />lower than Blaney-Criddle high-altitude estimates by about 5 percent for every analysis <br />year. <br />Potential ET provides only the upper limit for consumptive use -water supply is used to <br />estimate actual consumptive use. In addition, crop acreage above 6,500 feet in the South <br />Platte drainage is very small compared to crop acreage below 6,500 feet. Therefore, we <br />recommend the use ofBlaney-Criddle high-altitude crop coefficients above 6,500 feet for <br />the SPDSS efforts. In addition, we continue to recommend additional lysimeter studies <br />in high-altitude settings and recommend incorporating CoAgMet-type climate stations in <br />high-altitude agricultural settings in Colorado. <br />3. South Platte Plains Crop Coefficients <br />• Plains calibrated coefficients - Dr. Allen requested more information regarding the <br />quality control of daily CoAgMet and NCWCD climate data used in the Penman <br />Monteith estimate of ET (subsequently used to calibrate Blaney-Criddle coefficients). A <br />general quality control of the daily weather data was performed based on the procedures <br />outlined in Appendix D of the ASCE Standardization for Reference ET Manual. <br />• Growing Season Triggers - Dr. Allen suggested using local planting and harvesting <br />dates in the development and application of plains calibrated crop coefficients. Our <br />review of local information provided in the Colorado Irrigation Guide showed that local <br />Page 4 of 7 <br />