The ASCE Standardized Reference Evapotranspiration Equation

This Task Committee 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 Manual 70. Along with applications for 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 by the Environmental and Water Resources Committee (EWRI) of ASCE, was made at the request of, and has been endorsed by, the Irrigation Association.