Application of Penman Equation Wind Function

The Penman equation for computing evapotranspiration is frequently applied as the basis of irrigation system design. It has seen widespread application when sufficient data are available because of its sound theoretical basis. This equation combines an energy balance and an aerodynamic term. The aerodyamic portion of the equation contains a measure of vapor pressure deficit multiplied by an empirically derived wind function. Uncertainties and errors have arisen in the application of this equation because of misconceptions regarding calculation of the vapor pressure deficit. Derivation of the original Penman aerodynamic term is indicated along with recent modifications to the wind function. Correct methods of computing the vapor pressure deficit corresponding to various wind functions are shown. The importance of using wind function coefficients consistent with the method of computing the vapor pressure deficit for design applications is stressed.