Prepared by the Task Committee on Standardization of Reference Evapotranspiration of the Environmental and Water Resources Institute of ASCE.

This report provides standardized equations for calculating reference evapotranspiration (ET) from weather data and procedures for quality assessment and control of weather data. 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 agriculture and landscape use.

The basis of the standardized reference ET equation is the ASCE Penman-Monteith (ASCE-PM) method discussed in ASCE Manual of Practice 70, Evapotranspiration and Irrigation Water Requirements. Along with applications of the ASCE-PM method, this report includes recommended calculations for vapor pressure, net radiation and wind speed adjustment, and guidelines on assessing weather data integrity and estimating values for missing data.

The development of this standardized report was made at the request of, and has been endorsed by, the Irrigation Association.