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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #115462
Title: ASCE'S STANDARDIZED REFERENCE EVAPOTRANSPIRATION EQUATION

Author
item WALTER, I - W.W. WHEELER & ASSOC.
item ALLEN, R - UNIVERSITY OF IDAHO
item ELLIOTT, R - OKLAHOMA STATE UNIVERSITY
item JENSEN, M - SELF-EMPLOYED
item Howell, Terry
item SNYDER, R - UNIVERSITY OF CALIFORNIA
item BROWN, P - UNIVERSITY OF ARIZONA
item ECHINGS, S - DEPT WATER RESOURCES, CA
item SPOFFORD, T - USDA-NRCS, PORTLAND, OR
item Wright, James

Submitted to: American Society Of Civil Engineers Watershed Management Conference
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2000
Publication Date: N/A
Citation: N/A

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 standardized equation is derived from the ASCE-Penman Monteith equation by simplifying several terms within that equation. The standardized equation, with appropriate constants provided in an accompanying table, is used to calculate evapotranspiration for the standardized short reference (ETos) and/or evapotranspiration for the standardized tall reference (Etrs). One constant is in the numerator and one is in the denominator. The constant in the right-hand side of the numerator (Cn) is a function of the time step pand aerodynamic resistance (i.e., reference type). The 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 to 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 evapotranspiration; therefore, the Kcr*ETrs or Kco* ETos procedure will more readily be adopted by the private sector as well as federal and state agencies; 3) more universal hourly equations that will provide better comparisons between summed hourly reference ET and daily reference ET.