Evaluation of Estimation Methods for Monthly Reference Evapotranspiration in Arid Climates

Reference evapotranspiration ( ET 0 ) plays a key role in irrigation system design as well as water management of agricultural ecosystems under irrigated and rainfed conditions. While many methods for estimating the ET 0 have been developed during the past several decades, method selection essentially depends on the availability of measured climatic variables. The FAO-56PM method recommended by experts from Food and Agriculture Organization of the United Nations is widely used in agricultural and environmental research to estimate the ET 0 . However, it requires several climatic parameters that are not always available in developing countries, especially in arid regions. Here, we compare and evaluate the performance of 13 widely- and commonly-used equations for estimating ET 0 against that predicted using the FAO-56PM model using climatic data from nine meteorological stations located in arid regions across Iran. On average, the best three methods that could be used as an alternative to the FAO-56PM equation were the Irmak (Irmak et al., 2003), Hargreaves-Samani (Hargreaves and Samani, 1985), and Hargreaves (1975) equations.