Temperature- and Radiation Based Methods against the Standard FAO Penman- Monteith for Estimating the Reference Evapotranspiration (ET0) in Gorgan

The FAO Penman - Monteith (F-P-M) method is now broadly accepted as the standard for estimating the reference evapotranspiration (ET0). The method is also now used for evaluating the temperature-based [Blaney-Criddle (B-C) and Hargreaves-Samani (H-S)] and radiation-based [Jensen-Haise (J-H) and Turc (Tc)] methods. The objective was to compare the ET0 estimated by the B-C، H-S، J-H، Tc methods with that estimated by the F-P-M method in a semiarid climate using the meteorological data recorded in Hashem-Abad Agro-Meteorological Station، Gorgan. The mean value of ET0 was 3. 13 mm. day-1 using the F-P-M method. At daily scale، Tc method showed the lowest RMSE (Root Mean Square Error)، 0. 98 mm. day-1. At monthly scale، in compare to the other methods، the RMSE values by Tc method were lower in April as well as from September to March، averaged 0. 06 mm. day-1 with the percentage error of 3%. During the summer season، July and August، however، B-C method suggested the lowest RMSE value than those of the others (RMSE 0. 32 mm. day-1 with the percentage error of 6%). The H-S method behaves the best for monthly estimation (RMSE 0. 42 mm. day-1، the percentage error of 7%) during May to June. The F-P-M method for estimating ET0 requires data often not recording in the meteorological stations; hence ET0 should be estimated using the proposed simpler methods in compare to the F-P-M. The adaptation of the methods to different climates should be essentially tested by the standard F-P-M method.