COMPARISON OF DIFFENET HARGREAVES-SAMANI METHODS FOR ESTIMATING POTENTIAL EVAPOTRANSPIRATION IN ARID AND SEMI-ARID REGION OF IRAN

摘要

For calculating the Potential Evapotranspiration (ET0), it is necessary that standard models be designed for different climates. Among the numerous methods, the Hargreaves-Samani is one that is having a global validity for accuracy in estimating ET0 in arid and semi-arid regions. This study, compared different Hargreaves-Samani models which have been developed since 1982 till now. Grass evapotranspiration from lysimeters and climatologically data for 9 locations in 1988 to 1997 were used for this purpose. The results of this research show that if the climatology data are measured in irrigation field, it is better to use average minimum temperature else it must replace this parameter with dew point temperature (Td). In addition, Initial Hargreaves-Samani (1985) method estimated potential evapotranspiration accurately when average wind speed was equal or less than 1 meter per second, while with more than 1 meter per second the modified Hargreaves-Samani by Droogers and Allen (2002) calculated better than other methods. Also, if potential evapotranspiration was above 6 millimeter per day and when Td adjustments of these equations were used, the MARE and RMSD were decreased.