Advection Modification of the Priestley and Taylor Evapotranspiration Formula1

The empirical formula of Priestly and Taylor (1972) relating the evapotranspiration from a well-watered surface to the net radiation and a function of the air temperature is modified to include a saturation deficit term to account for high local advection. The model is tested on two seasons of daily evapotranspiration measurements over irrigated potatoes (Solarium tuberosum L.), resulting in good agreement and a substantial improvement over the unmodified formula. Alfalfa (Medicago saliva L.) measurements are also represented well by the model, but no better than by the Priestley-Taylor formula calibrated during a period of high advection.