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Weather station wind speed data for use in the ASCE Standardized Reference Evapotranspiration Equation are expected to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. Many agricultural weather stations used for evapotranspiration estimation may not meet such siting conditions for all or part of the growing season. A physically-based 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 and 2009 growing seasons at 2-m and 3-m heights above ground surface over and downwind of variable height alfalfa in several southeast Colorado locations. These wind speed measurements were translated to estimated wind speed at 2 m over grass, and compared. The effects on computed reference evapotranspiration are illustrated and discussed. The results have important consequences in determination of the State of Colorados compliance with the Arkansas River Compact.