SPDSS_Task59-1_Develop Locally Calibrated Blaney-Criddle Crop Coefficients

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Table 11 Average Annual PCU Estimates Using Upper Plains Calibrated Coefficients at Fort Lupton and Lower Plains Calibrated Coefficients at Sterling Crop Type Ft. Lupton 1993-2003 Sterling 1996-2006 Alfalfa 2.66 2.93 Corn Grain 1.81 2.08 Dry Beans 1.36 1.53 Grass Pasture 2.86 3.21 Small Grains 1.73 1.95 Sugar Beets 2.09 2.17 Monthly time series plots (Figures 29 through 40 provided at the end of this memo) for the six crops at Fort Lupton using the Upper Plains coefficients and at Sterling using the Lower Plains coefficients for the available period of record show that the PCU estimates using the calibrated coefficients closely approximate the ASCE Standardized method with the Wright crop coefficients. Comments and Concerns The recommended calibrated coefficients (Tables 5 through 10) will be added to the SPDSS StateCU input dataset and HydroBase. Following is a summary of the results from the locally calibrated Blaney-Griddle crop coefficients created under this task: • For high altitude pasture grass, the recommended coefficient adjustments result in over 30% more PCU than unadjusted values (Table 2). • For alfalfa, grass pasture, and small grains, the recommended coefficient adjustments result in approximately 0% to 28% more PCU than the unadjusted values (Table 4). • For dry beans and sugar beets the recommended coefficient adjustments result in approximately 0% to 23% less PCU than the unadjusted values (Table 4). • For corn grain, the recommended coefficient adjustments result in approximately the same PCU (ranging from 9% more to 6% less PCU) as the unadjusted values (Table 4). This memorandum has been updated to reflect comments and questions raised at an SPDSS Consumptive Use and Water Budget Technical Peer Review meeting and additional analyses conducted in response to that meeting. Following are comments and concerns that reflect this additional review: The South Park-Walter study showed that the season of significant plant growth was well-approximated by using afive-day temperature threshold of 42 degrees. For SPDSS, the South Park-Walter coefficients will be used with a growing season defined by a mean temperature of 42 degrees for both spring and fall as an approximation. Different PCU estimates above and below an elevation of 6,500 feet may result if the South Park -Walter study lysimeter-derived crop coefficients are used for the high Task59_l.doc (updated January 11, 2008) 20 of 32