Optimum Correction Factor for Hargreaves- Samani Method for Determination of Reference Crop Evapotranspiration (Case study: Khorasan Razavi Province)

Reference Crop Evapotranspiration (ETo) is one of the most important component of hydrologic cycle. It is estimated either through lysimetriy or by empirical equations. Hargreaves-Samani (H-S) is one of the simplest methods that needs only to temperature data (minimum and maximum daily). The purpose of this paper is to find the best correction factor applying to H-S method that translates to the assumed true Fao Penman Monthis method for synoptic stations of Khorasan Razavi Province. Such a correction factor, however, is taken constant in nearly all of the published papers, which demands for local-nature. Here, it is considered as a function of those meteorological variables available at evaporation stations, considering temperature and humidity ones, and some regression equations were derived. The parameters of these equations, based on the results, were not constant for different synoptic stations, which confirms for non-constant correction factor. Different equations were ranked based on 7 different evaluation methods and the best equation was found for each synoptic station. Influenceregion for each synoptic station was determined and evaporation stations within each region were determined.