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MEMORANDUM December 13, 2013 Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney-Criddle Eqn. Page 20 of 34 PART III: EVALUATION OF TASK MEMO 59.1 Background: Task Memo 59.1 presented a methodology for calibrating the SCS Modified Blaney-Criddle equation to the ASCE Standardized Reference ET Equation28 (herein referred to as “The ASCE”). This methodology will often increase the historical consumptive use of a water right when compared to TR-21. An analysis using data from Kersey, CO was performed to show how applicable the upper plains coefficients for pasture grasses are to a specific site, such as in a change of use analysis. This evaluation also compares the findings of Task Memo 59.1 to those of Pochop using an elevation adjustment, and published Blaney-Criddle coefficients from ARS 1275. In addition, an independent calibration was performed in attempt to duplicate the results of Task Memo 59.1. The upper plains coefficients were developed by calibrating the SCS modified Blaney-Criddle equation to The ASCE. The two equations were solved independently for a given month using CoAgMET data from the same climate station. The kc value was determined by dividing The ASCE results by the SCS TR-21 results. The resulting factor was then applied to the kc value and the process was repeated. A summary of the input data for the analysis is provided below. SCS TR-21 Blaney-Criddle ASCE Stnd. Ref. ET Equation Type of Climate data (1993 - 2003) Monthly29 Daily Source of crop (growth stage) coefficients TR-21 Table 6.9 of Manual 70 30 Loc. where coefficients were developed (unknown, varied) Kimberly, ID Growing season parameters TR-21 TR-21 Reference Crop N/A Tall (f/k/a, alfalfa) Table 16 – Input data for calibration procedure in Task Memo 59.1 It is important to note that Manual 70 references the ASCE Penman-Monteith Equation for ET and Table 6.9 has not been adjusted for use with The ASCE. A small adjustment should be made to these coefficients before using them with The ASCE. This adjustment has been done by Dr. Richard Allen and can be found in Chapter 8, Irrigation Requirements of Design and Operation of Farm Irrigation Systems31 (herein referred to as “Chapter 8”). The SPDSS defines the upper plains region as District 1, 2, and the lower portions (below 6500’ elevation) of 3, 4, 5, 6, 7, 8, and 9. However, the upper plains coefficients really represent an average of Ft Collins, Ft Lupton, and Greeley since that is where the data came from. Climate data from these three locations were used to develop independent coefficients, which were then averaged. Ft Collins and Greeley are located in District 3 and Ft Lupton is located in District 2. No data from Districts 1, 4, 5, 6, 7, 8, or 9 were considered. The upper plains coefficients may have been adequate for the basin-wide planning that the CWCB was involved in, but may not represent a particular site within the upper plains basin. Since the data used was from Ft Collins, Ft Lupton, and Greeley the results may only accurately represent that area in a site specific analysis. An example of this can be seen when evaluating pasture grass in Kersey, CO. Kersey Analysis (pasture grasses): Kersey is located approximately 10 miles east-southeast of Greeley, CO. Climate data from CoAgMET (KSY01) were used to calculate monthly consumptive use of pasture grasses using the SCS modified Blaney-Criddle equation with TR-21 coefficients, the ASCE Standardized Equation, and SCS modified Blaney-Criddle equation with the upper plains coefficients from Task Memo 59.1. The monthly results were plotted from 1993 to 2008 and show that for pasture grasses in Kersey, CO the upper plains coefficients over estimate consumptive use when compared to the ASCE method and SCS TR-21 method. Kersey, CO may not be represented well by Ft Collins, Ft Lupton, or Greeley climate data. 28 Allen, et al. (2005) 29 Average monthly temperature was calculated from daily max & min temperature 30 Reported mean crop coefficients were used 31 Allen, et al. (2007)