Adjusting Wind Speed Measured over Variable Height Alfalfa for Use in the ASCE Standardized Penman-Monteith Equation

The ASCE Standardized Reference Evapotranspiration Equation expects the weather station wind speed data to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. The Task Committee on the Standardized Equation provided guidance for adjusting wind speed measured at height other than 2 m, or, for situations when the wind speed is measured over and downwind of 0.5 m alfalfa. The latter adjustment attempts to account for the effects of both grass and alfalfa crop characteristics (height, roughness) on the wind profile. A more physically representative approach to adjust wind speeds at various heights and various weather measurement surface conditions to equivalent wind speed at 2 m height over clipped grass is tested. Wind speeds were simultaneously measured during the 2008 growing season at 2-m and 3-m heights above ground surface over variable height alfalfa at two Colorado Agricultural Meteorological Network (CoAgMet) electronic weather stations and at the research alfalfa lysimeter installation at the Colorado State University Arkansas Valley Research Center. These wind speed measurements were adjusted to estimated wind speed at 2 m over grass, and compared.