Submitted to: Complete Book
Publication Type: Book / Chapter
Publication Acceptance Date: 11/15/2004
Publication Date: 6/15/2005
Citation: Allen, R.G., Walter, I.A., Elliott, R.L., Howell, T.A., Itenfisu, D., Jensen, M.E., Snyder, R.L. 2005. The ASCE standardized reference evapotranspiration equation. Reston, VA:American Society of Civil Engineers. 59 p. Interpretive Summary: The concept of reference evapotranspiration (ET) was developed in the 1970’s as a practical and definable replacement for the term potential ET. Reference ET is a function of local weather, represents the crop water use from a defined vegetated surface, and serves as an evaporative index by which engineers, hydrologists, water managers and other technical professionals can predict ET for a range of vegetation and surface conditions by applying “crop” coefficients for agricultural or landscaped areas. During the past decade, for convenience and reproducibility, the reference surface has been expressed as a hypothetical surface having specific characteristics. In the context of this standardization, reference evapotranspiration is defined as the ET rate from a uniform surface of dense, actively growing vegetation having specified height and surface resistance, not short of soil water, and representing an expanse of at least 100 m of the same or similar vegetation. The American Society of Civil Engineers-Environmental Water Resources Institute (EWRI) Task Committee concluded that two standardized surfaces were needed to serve the needs of the agricultural and landscape communities and to provide for continuity with past reference ET usage. This recommended standardization follows commonly used procedures for calculating the reference ET for each of the reference surfaces using a single equation having fixed constants and standardized computational procedures. The computational procedures are relatively simple to apply, are understandable, are supported by existing and historical data, are technically defensible, and are accepted by science and engineering communities. The development of this standardization report by EWRI was made at the request of, and has been endorsed by, the Irrigation Association.
Technical Abstract: This report describes the standardization of calculation of reference evapotranspiration (ET) as recommended by the Task Committee on Standardization of Reference Evapotranspiration of the Environmental and Water Resources Institute of the American Society of Civil Engineers. The purpose of the standardized reference ET equation and calculation procedures is to bring commonality to the calculation of reference ET and to provide a standardized basis for determining or transferring crop coefficients for agricultural and landscape use. The basis of the standardized reference ET equation is the ASCE Penman-Monteith (ASCE-PM) method of ASCE Manual 70. For the standardization, the ASCE-PM method is applied for two types of reference surfaces representing clipped grass (a short, smooth crop) and alfalfa (a taller, rougher agricultural crop), and the equation is simplified to a reduced form of the ASCE–PM. Standardized calculations for vapor pressure, net radiation and wind speed adjustment are recommended for application to hourly and daily calculation time steps. Guidelines on assessing weather data integrity and estimating values for missing data are provided.