Spatial and temporal calibration of Hargreaves equation for calculating monthly ETo based on Penman‐Monteith method

The Penman-Monteith equation is the most common method for estimating reference crop evapotranspiration (ETo). However, this method needs full weather data, but few stations with complete weather data exist in Fars province in Iran. On the other hand, the Hargreaves equation is a simpler method for estimating ETo. In this study, using the weather data of seven synoptic stations in Fars province in Iran and seven synoptic stations outside the province, the Hargreaves equation was calibrated based on the Penman-Monteith method for every month of the year and for a yearly time step. The results indicate that different coefficients should be used for each month of year and yearly time step instead of the original coefficient of 0.0023 in the Hargreaves equation. Therefore, it is possible to estimate monthly ETo by using the calibrated coefficients of the Hargreaves equation for different regions of Fars province for each month of the year. Also, according to the geographical locations of each station and by using the GIS-assisted methods, spatial distribution maps of yearly calibrated coefficients in Fars province were prepared by the inverse distance square method. Copyright © 2007 John Wiley & Sons, Ltd.