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Appendix <br />A Brief Look at the ASCE Standardized Penman-Monteith Equation <br />The Penman-Monteith method was ranked as the number one (most accurate) method for <br />potential consumptive use determinations in a detailed review presented by Jensen (1970). The <br />primary required inputs to the 1965 implementation of the Penman-Monteith equation include <br />temperatures (maximum and minimum air, and dewpoint), solar radiation, vapor pressure and <br />wind speed. Jensen describes many secondary equations (i.e. air density, latent heat of <br />vaporization, canopy resistance) used in the Penman-Monteith calculation. The result is a very <br />technical and complicated calculational process to arrive at aPenman-Monteith consumptive use <br />value. <br />In 1999, the Irrigation Association requested a technical committee of the American Society of <br />Civil Engineers (ASCE) that is involved with evapotranspiration to propose one standardized <br />equation and set of procedures for calculating evapotranspiration. The goal was a consistent <br />model that would have wide acceptance. A subcommittee (task force), chaired by Ivan Walter of <br />Colorado, initiated meetings and debates to arrive at a standardized equation. The result of this <br />process was that two equations (one for a short crop named ETo and one for a taller crop named <br />ETr) were developed. These equations were based on framework suggested in a FA056 (Allen, <br />1998) document that assumed a constant for the psychrometric constant, simplified the air <br />density term and simplified the vapor aerodynamic resistance term. For each type of reference <br />crop (short or tall), constants were assigned in the ASCE Standarized P-M equation for the <br />vegetation height, latent heat of vaporization, and surface resistance. The resulting equation for <br />ET (for both short and taller crops) is: <br />Cn <br />408 4(R„ - G) + y ---------- U2 (es°-ea) <br />T+273 <br />ETSZ= ----- <br />4 + y(1+CdU2) <br />Where: <br />ETSZ= standardized reference crop evapotranspiration for a short or tall reference crop <br />4 =slope of saturated vapor pressure curve <br />R„ =net radiation flux <br />G =sensible heat flux into the soil <br />y =psychrometric constant <br />C„ =numerator constant for the reference crop type and time step (see Table Al below) <br />T =temperature (usually daily mean air temperature) <br />UZ =wind speed <br />es° =mean saturated vapor pressure <br />ea =mean daily ambient vapor pressure <br />Cd = demonimator constant for the reference crop type and time step (see Table Al below) <br />