EVALUATION OF REFERENCE EVAPOTRANSPIRATION USING REAL AND ESTIMATED SUNSHINE HOURS IN A SEMI-ARID AND ARID ENVIRONMENT

Reference crop potential evapotranspiration (ET0) is a major component of the hydrological cycle, and its precise determination has a high importance in the studies of water balance design and management of irrigation systems, product yield simulation, and design and management of water resources. In this study, an area in the center of Iran (about 12% of the total area of Iran) with latitude (N 32 degrees - 35 degrees) and semi-arid and arid climates was chosen for estimation of monthly sunshine hours (n) based on weather data (temperature and humidity), and the obtained equations were used for estimating monthly ET0 based on the Penman-Monteith equation (FAO-56 PM). The results were compared with the Penman-Monteith equation (a reference equation) by using recorded sunshine hours data. The results indicated that the derived equation (n = 7.976 + 0.160T(max) - 0.072T(min) - 0.022RH(min) - 0.036RH(max)) for estimating sunshine hours had high accuracy with MAE = 0.083 mm.d(-1), RMSE = 0.139 mm.d(-1) and R-2 = 0.998. The obtained results showed very good agreement with observed data from a similar climate. Therefore, it is possible to estimate monthly ET0 with FAO-56 PM equation in similar areas without using the recorded hourly sunshine.