Evaluation of alternative methods for estimating reference evapotranspiration

Abstract

Evapotranspiration is an important component in water-balance and irrigation scheduling models. While the FAO -56 Penman-Monteith method has become the de facto standard for estimating reference evapotranspiration ( ET o ), it is a complex method requiring several weather parameters. Required weather data are oftentimes unavailable, and alternative methods must be use d. Three alternative ET o methods, the FAO -56 Reduced Set, Hargreaves, and Turc methods, were evaluated for use in Mississippi, a humid region of the USA, using only measurements of air temperature. The Turc equation, developed for use with measured temperature and solar radiation, was tested with estimated radiation and found to provide better estimates of FAO -56 ET o than the other methods. Mean bias errors of 0.75, 0.28, and -0.19 mm, mean absolute errors of 0.92, 0.68, and 0.62 mm, and percent errors of 22.5%, 8.5%, and -5.7% were found for daily estimates for the FAO -56 Reduced Set, Hargreaves, and Turc methods, respectively.