Translating Wind-Speed Measurements over Alfalfa Having Varying Height for Use in the ASCE Standardized Reference ET Equation

The ASCE Standardized Reference Evapotranspiration (ET) Equation expects wind speed measured at a height of 2 m over and downwind of a smooth measurement surface, such as actively growing clipped grass. Many agricultural weather stations used for evapotranspiration estimation may not meet such siting conditions. A physically based approach to translate wind speeds measured at various heights and weather measurement surface conditions to equivalent wind speed at 2-m height over actively growing clipped grass is tested in agricultural settings. The translation algorithm assumes neutral or near-neutral boundary layer stability conditions. Mean daily wind speeds measured at 2- and 3-m heights above ground surface over and downwind of variable-height alfalfa during the 2008, 2009, and 2010 growing seasons in several southeast Colorado locations were translated to equivalent mean daily wind speed at 2-m height over clipped grass and compared. Translated alfalfa weather station mean daily wind speeds were also compared with mean daily wind speeds measured at 2-m height over 0.12-m grass for parts of the 2009 and 2010 seasons at two pairs of sites. The translation algorithm produced translated mean daily wind speeds in good agreement with wind speeds measured under the standard weather station condition. The effects on computed daily tall crop reference evapotranspiration are illustrated.