SPDSS_Task59-1_Develop Locally Calibrated Blaney-Criddle Crop Coefficients

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7B Lauver Plains ~~ Region ~f '< 48 !y ~ ' 3 ~ ~~ ' ~ 4 7 ~ 3 L ~~ ~ ~ 64 ~ ~~ • r ~ ~;- • Fort Co I s `~^~~ Sterling ~ '' ,._ 4 ~ 4 L r eley w_ i . ~~It~'r..-! 1 S. High AI#ifude ''5 U 5 L } I ~ L Region ~ ~ •Fo u on pt ,;'6U 6L 2 7 L 7U Upper Plains g U 9 Region __ , ~cl Legend R; a U 23 0 Water Districts [ u =upper Portion ] [ L= Lower Portion ] ~~~ Elevation [ft] ~~ ~ ~~ ~ High :1'482 ti ^.. Low:3285 • ^aily Climate Stations _ Calibrated Coefficient t,~'~ _ u ~ _~ ~ ;_ ~~ Geographic Regions Figure 4 -Calibrated Coefficient Geographic Boundaries 4. Develop Locally Calibrated Crop Coefficients 4.1 High Altitude Portions of the SPDSS Study Area Lysimeter-derived crop coefficients, because they represent actual field experiments, produce more accurate results than using a generalized consumptive use method with standardized crop coefficients. Even if an adjustment for elevation is made to the generalized method and standardized coefficients, consumptive use results are not typically as accurate as those based on local lysimeter data. As an example, the original Blaney-Criddle consumptive use method was analyzed using the lysimeter-derived coefficients from the Walter study (Table 1) and compared to the modified Blaney-Criddle method run with the TR-21 crop coefficients and an upward elevation adjustment of 10% for each 1,000 meters increase in elevation above sea level (as recommended in ASCE, 1990). While PCU is a linear function of the crop coefficients with the original Blaney-Criddle method, the relationship is not linear with the modified Blaney-Criddle method due to the additional temperature adjustment. Therefore, crop coefficients for the two methods cannot be compared directly. However, the resulting PCU of pasture/meadow grass can be compared and are provided in Table 2 at two high altitude climate stations, Antero Reservoir and Bailey. Task59_l.doc (updated January 11, 2008) 7 of 32