Calibration of hargreaves equation for estimating monthly reference evapotranspiration in the West of Iran

In the present study, the Hargreaves-Samani method was calibrated by the Penman-Monteith (PM) equation under semi-arid conditions in the western region of Iran. Different correction coefficients were used for each station under study instead of the constant coefficient 0.0023 in the Hargreaves equation based on monthly and annual scales. The corrected coefficients obtained for different regions can be used to estimate ETo for the PM method under the same climates where reliable data are available. The results also showed that as a larger study area with more weather stations is used and a larger amount of data is obtained, the coefficient of 0.0023 in the Hargreaves equation does not need any further corrections for local calibration purposes.