Evaluation of Reference Evapotranspiration Equations in Semi-arid Regions of Northeast of Iran

ABSTRACT: Accurate estimation of evapotranspiration is required for ef?cient irrigation management, estimation of plant water and design of irrigation systems. Due to complexity and interaction of meteorological parameters, accurate calculation of ET by physical or combination method such as Penman–Monteith (PM) equation requires many input data, which are usually not available. Therefore, many empirical equations have been introduced for estimation of ET0 in different regions of the world. But the method used in other regions require evaluation before application and if necessary optimizations of parameters is carried out. Hargreaves – Samani, Blaney – Criddel, Priestley – Taylor, Turc, Thornthwaite and Makkink are the most common equations for calculation of evapotranspiration. The objective of this study is to evaluate ETo estimated by these equations against the corresponding values estimated using the standard Penman – Monteith – FAO equation for the Bojnord synoptic station. For this purpose, ETo estimates by the all equations were statistically compared with the Penman – Monteith – FAO estimates. The method which produced the smallest difference with the Penman – Monteith – FAO, selected as the best equation for estimation of evapotranspiration in the North Khorasan province. Results indicated that all equations used in this study show a good performance, despite their level of complexity and data requirement. The Hargreaves – Sammani showed the smallest difference in comparison with the Penman – Monteith – FAO method and the Makkink equation is the second. The Blaney – Criddle presented the worst results.