Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: 11/14/2000
Publication Date: N/A
Citation: Interpretive Summary: Evapotranspiration (ET) is the combined process of evaporation from soil and plant surfaces and transpiration through the plant stomata. It is widely used in irrigation scheduling, modeling crop production, and in determining the legal water rights in many states. Several methods have been used to compute ET, and often methods vary across various agencies. This paper discusses the need for a Standardized Reference Evapotranspiration Equation and explained the equation developed by a panel of experts to serve the agriculture and irrigation industry. The equation was selected based on the criteria that it be understandable, defensible, and simple, accepted by science/engineering communities, and facilitate the use of existing data and technology. Based upon comparisons to lysimeter data and calculated reference evapotranspiration using 1982 Kimberly Penman equation, the FAO-56 Penman Monteith equation, and the ASCE Penman Monteith hequation, the standardized equation was sufficiently accurate to recommend its use for calculation of reference evapotranspiration, the development of crop coefficients, and estimation of crop evapotranspiration.
Technical Abstract: 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 (Cn) is a function of the time step and aerodynamic resistance (i.e., reference type). A constant in the denominator (Cd) is a function of the time step, bulk surface resistance, and aerodynamic resistance (the latter two terms vary with reference type, time stop 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; and 3) more universal hourly equations that will provide better comparisons between summed hourly reference ET and daily reference Et.