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

An accurate estimation of an evapotranspiration is required for an efficient irrigation management, estimation of plant water and design of an irrigation systems. Due to complexity and interaction of meteorological parameters, an 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. 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 -- FAQ method and the Makkink equation is the second. The Blaney -- Criddle presented the worst results.