Abstract

The Penman-Monteith (PM) method is the standard equation for estimating reference crop potential evapotranspiration (ET_o) for different climates of the world. However, this equation needs full weather data, but few stations with complete weather data exist in Fars province, south of Iran. On the other hand, the Blaney-Criddle (BC) equation is a simpler alternative for estimating ET_o compared with the PM equation. In this study, by using the meteorological data of seven synoptic stations inside the Fars province, the modified BC equation which included the effective temperature was evaluated based on the PM equation for every month of the year. The effective temperature can be calculated using the approach proposed by other investigators based on the minimum and maximum temperature and a calibrated coefficient (k). This coefficient was calibrated spatially for different months in the Fars province in another study. However, in this study the variable k values, and three constant k values (0.72, 0.69 and 0.64) that was reported by other investigators was used to evaluate the modified BC equation. The results demonstrated that using variable k values for considering the effective temperature in the modified BC equation was better than using mentioned constant values. Therefore, the results emphasized the superiority of using variable kvalues for considering the effective temperature.

Key words: Blaney-Criddle equation, Penman-Monteith equation, Iran, evapotranspiration, effective temperature, fars province.