SPDSS_Task58_Review Previous Estimates of Potential CU

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SPDSS Task 58 Memo August 5, 2004 Page 8 of 22 currently in use by representatives of the SEO (it is included in DWR TR21 spreadsheet model) and consultants for calculation of lawn grass consumptive use in the South Platte River Basin and other parts of the state. 2. "Other" Methods of Consumptive Use Calculation Though the modified Blaney-Criddle (with or without an elevation adjustment) has been the predominant method employed by engineering consultants and Division of Water Resources representatives, several other methods or combinations of methods, as described below, were found, through the interviews, to be gaining favor. Penman Equations The Penman methods are generally recognized as more accurate, though much more data intensive, than the modified Blaney-Criddle method. Data requirements can include daily temperature, wind, solar radiation, and vapor pressure measurements. In the evaluation of consumptive use methods presented in the ASCE Manual 70 (Jensen, 1970), the first ranked (best) equation was the Penman-Monteith and the second ranked equation was the Kimberly- Penman. These rankings compare to the 15th rank of the SCS Blaney-Criddle method. W.W. Wheeler consulting engineers has on several occasions applied aKimberly-Penman consumptive use method including when estimating PCU use under the Fisher Ditch in the Clear Creek Basin, a tributary to the South Platte River or in rebuttal of PCU calculations made for Thornton's Northern Project in the Cache la Poudre Basin, a tributary to the South Platte River Basin. The Northern Colorado Water Conservancy District historically (before 1998) applied the Kimberly-Penman method in preparing estimates of daily evapotranspiration (see memo section on Daily Consumptive Use Estimates). They have since transitioned those calculations to a method referred to as the ASCE Standardized Penman-Monteith equation (see next paragraph). In an April 8, 2004 conference on "Evapotranspiration, Determination of Consumptive Use in Water Rights Proceedings", speakers (engineers and attorneys) emphasized the value of a more accurate, and detailed, method of PCU calculation. The method proposed by the speakers to be adopted for the determination of the PCU in Colorado was the ASCE Standardized Reference Evapotranspiration Equation. This equation was developed by the ASCE Environmental and Water Resources Institute Evapotranspiration in Irrigation and Hydrology Committee after that committee was asked to propose a standardized equation and set of procedures for estimating PCU. The purpose of having a standardized equation was to lend consistency and wider acceptance of ET models. This committee recommended two equations (one for a short reference crop (grass) and one for a tall reference crop (alfalfa)) based on the Penman-Monteith Equation. In the development of the standardized equations, the original Penman-Monteith equation was simplified through the addition of specified constants. Additional description of the ASCE Standardized P-M Equation is provided in the Appendix to this report.