ASCE's standardized reference evapotranspiration equation

The ASCE Evapotranspiration in Irrigation and Hydrology Committee (ASCE-ET) is recommending for the intended purpose of establishing uniform evapotranspiration (ET) estimates and transferable crop coefficients, two standardized reference evapotranspiration surfaces: (1) a short crop (similar to grass) and (2) a tall crop (similar to alfalfa), and one standardized reference evapotranspiration equation. The equation is a simplified ASCE-Penman Monteith equation. The equation with appropriate constants is used to calculate standardized reference ET, short (ETos) or standardized reference ET, tall (ETrs). A constant in the numerator (C-n) is a function of the time step and aerodynamic resistance (i.e., reference type). A constant in the denominator (C-d) is a function of the time step, bulk surface resistance, and aerodynamic resistance (the latter two terms vary with reference type, time step and daytime/nighttime). The standardized reference evapotranspiration surfaces and equation will provide: 1. A standardized calculated evaporative demand that can be used in developing transferable crop coefficients. 2. A clear methodology for practicing engineers to use for estimating reference ET. 3. More universal hourly equations that will provide better comparisons between summed hourly reference ET and daily reference ET. The equation was selected based on the criteria that it would be understandable, defensible, accepted by science/engincering communities, simple, and would enable using existing data and technology. Based upon comparisons to lysimeter data and calculated reference evapotranspiration using 1982 Kimberly Penman, FAO-56 Penman, and ASCE Penman Monteith, ASCE-ET found the standardized reference ET equation to be sufficiently accurate to recommend its use for calculation of reference ET, the development of crop coefficients, and estimation of crop evapotranspiration. ASCE recommends using the symbols K-co for crop coefficients to be used with short crop reference ETos, and K-er for crop coefficients to be used with tall crop reference ETrs.