A comparison of the Priestley-Taylor and Penman methods for estimating reference crop evapotranspiration in tropical countries

An equation for Potential Evaporation (PE) proposed by Priestley and Taylor in 1972 has fewer data requirements than the well established Penman Potential Transpiration (Et) equation. From their definitions, PE and Et values should both provide acceptable estimates of Reference Crop Evapotranspiration (ETo), as defined by Doorenbos and Pruitt. Analysis of mean monthly climatic data from 30 tropical stations, widely spread within the latitude zone 25°N to 25°S, showed that PE and Et estimates agreed closely when monthly rainfall exceeded monthly Et. The minimum data requirements for the Priestley-Taylor equation are daily net radiation and mean air temperature. The Penman equation additionally requires daily data for humidity and run of wind. As reliable field net radiometers become more widely available, the Priestley-Taylor PE equation offers a satisfactory alternative to the Penman Et equation for estimating ETo in humid tropical climates.