AbstractThe Penman–Monteith equation is the most common equation for estimating reference crop evapotranspiration (ETo). However, this equation needs full meteorological data, but few meteorological stations with complete meteorological data exist in Fars province, in the south of Iran. On the other hand, the Blaney–Criddle equation is a simpler alternative for estimating ETo compared with the Penman–Monteith equation. In this study, the Blaney–Criddle equation was calibrated based on the Penman–Monteith equation for every month of the year by using the meteorological data of seven synoptic stations inside and seven synoptic stations outside Fars province. For this purpose, the calibration coefficients of the Blaney–Criddle equation (a and b) were determined for all months at different stations, separately. Furthermore, in a new approach the mean monthly temperature in the Blaney–Criddle equation was replaced with the effective temperature for each station in the study area. The ETo values based on the new approach of the Blaney–Criddle equation were better than the results of the original Blaney–Criddle equation when compared to the Penman–Monteith equation as the reference equation. The calculated values of a were negative for all months and stations, and ranged between −4.84 and −0.84. Lowest a values were obtained in May, July and June, and the highest a values were obtained in December, November and January, i.e. the values of a were greater in cold months than warm months of the year. The values of b were positive for all months and stations, and ranged between 0.71 and 1.89. December, January and November showed the lowest b values while June, July and August showed the highest. Therefore, the values of b were greater in warm months than cold months of the year. Moreover, the range of variation of a values was larger than for the b values which implied that the a values have more impact on the estimations of ETo. Copyright © 2008 John Wiley & Sons, Ltd.