SPDSS_Task81-2_Consumptive Use and Water Budget Technical Peer Review Meeting Follow-Up

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precipitation. The effect on PCU will be discussed at a subsequent Technical Peer Review Meeting. 2. Grass Pasture High Altitude Crop Coefficients Data Quality -During his initial review, Dr. Allen did not have access to the Walter Study (Walter, LA., Siemer E. G., Quinlan J. P., and Burman, R.D. "Evapotranspiration and Agronomic Responses in Formerly Irrigated Mountain Meadows, South Park, Colorado", Report for the Board of Water Commissioners, City and County of Denver, CO. March 1, 1990.) The Walter Study developed high-altitude Blaney-Criddle coefficients based on lysimeter data from several sites in Colorado. Dr. Allen was looking for more information about how the lysimeters were maintained, reviewed and data adjusted, as necessary, to account for differences in vegetation height, surrounding field conditions, etc. The SPDSS Task 59.1 memorandum included a detailed summary of the quality control of data performed for the Walter Study. Our review indicated the Walter Study included a reasonable effort to identify and exclude lysimeter data that was not reflective of irrigated meadows and adjusted the data, as necessary, for consistency. We agree with Dr. Allen that use of a combination method such as the Penman Monteith to calibrate the lysimeter data may produce more consistent and physically based PCU values. However, the Blaney-Criddle method was used in all of the historical high altitude lysimeter studies due to lack of daily climate data required for the more detailed methods. Therefore, we consider the Walter Study coefficients and the Blaney-Criddle method appropriate for estimating PCU over the extended study period. Growing Season Triggers -The SPDSS Task 59.1 Memorandum recommended that lysimeter-derived coefficients from the South Park study by Walter et al. be used with the original Blaney-Criddle method for high altitude areas of the SPDSS, using the growing season trigger of 42 degrees mean temperature recommended in the study. The South Park study by Walter et al. found growing started upon a S-day mean temperature of 42 degrees. However, the original Blaney-Criddle method compares a growing season trigger to mean monthly temperature, interpolating between mean monthly temperature values to determine the day of the month upon which growing starts, and then prorating the monthly PCU estimate by the number of growing days. To be consistent with the original Blaney-Criddle method, we recommended using a mean monthly value of 42 degrees as the trigger. Dr. Allen suggested using local planting and harvesting dates. Our review of local information provided in the Colorado Irrigation Guide showed that local planting and harvest dates can vary widely. We believe the growing season temperature-based parameters outlined in TR-21 are appropriate for the regional estimates being modeled under SPDSS. • Shoulder Month Coefficients -The high-altitude crop coefficients recommended in the Walter Study do not include crop coefficients for April. At some high-altitude climate stations, the mean monthly temperature may reach 42 degrees in April in a few years, but Page 3 of 7