Correction factor for Hargreaves-Samani method to estimate ETo (Case Study: Gorgan Synoptic Station)

The evapotranspiration (ET) is the one of the most important component of hydrological cycle. Nowadays many researchers presented the valuable relations involving climatic parameters for estimating the potential evapotranspiration of reference plant (ETo), prevent from water stress and water loss. Among them the Hargreaves-Samani (H-S) method is common in the regions that the measuring of meteorological parameters are limited. This study aims to determine the most accurate correction factor for H-S method to estimate ETo according to the FAO-Penman-Monteith (F-P-M) method in Gorgan synoptic station. This coefficient is related to climatic parameters which are available in climatology stations. At first equations were optimized using all parameters in excels environment, then some less effective parameters were omitted and again more equations optimized using the others. For verification of results, the meteorological data were divided to 2 groups namely 80 and 20 percent for calibrating and testing the model respectively. The results showed that the equation which included the maximum and average of daily temperature and max, min and average of relative humidity (RH) data, had the maximum ranking due to statistical investigation (R=0.93, RMSE=0.29).Using the mentioned equation the mean annual evapotranspiration of 1165mm (using H-S method) reduced to 925mm.Moreover there was lower error compared to the F-P-M- method which estimated 853 mm of evapotranspiration.