Equation for Evaporation Pan to Evapotranspiration Conversions

The evaporation rate from a class "A" evaporation pan located in a site surrounded by low-growing vegetation can be used to estimate potential evapotranspiration for a grass reference. The evaporation rate for pans and the evapotranspiration rate of grass differ depending on siting and weather conditions. Weather-based pan coefficients for the conversion of pan evaporation to evapotranspiration of a tall grass reference crop have been published and equations have been developed to estimate the published coefficients. However, the equations used to estimate pan coefficients are complex. A simpler equation to estimate potential evapotranspiration for a grass reference from evaporation pan data, upwind fetch of low-growing vegetation, mean daily wind run, and mean daily relative humidity is presented. The equation can be used to automate the computation of evapotranspiration from electronically recorded evaporation pans or to analyze historical evaporation pan data. The equation can be substituted for more complex equations or for tabular evaporation pan coefficients presented in Doorenbos and Pruitt's 1977 publication FAO Irrigation and Drainage Paper 24.