An evaluation of common evapotranspiration equations

A comparison is made between the Pruitt and Doorenbos version of an hourly Penman-type equation, the Food and Agriculture Organization (FAO) hourly Pe nman-Monteith equation, and an independent measure of reference evapotransp iration (ET,) from lysimeter data. Reducing the canopy resistance improved the hourly FAO Penman-Monteith estimates. Daytime soil heat flux density is estimated as 10% of net radiation in the FAO hourly Penman-Monteith equati on, however, the measured soil heat flux density under grass that was never shorter than 0. 10 m in this study was between 3% and 5% of net radiation. The daytime totals of hourly ET, from the hourly Penman-Monteith and Pruit t-Doorenbos equations and ET, from the 24-h FAO Penman-Monteith equation we re computed using data from five Italian and five Californian stations. A c omparison showed that all of the equations gave acceptable results. The Pru itt-Doorenbos equation may slightly over-estimate ET, in conditions of summ ertime told air advection.