EVALUATION AND CALIBRATION OF THREE EVAPOTRANSPIRATION EQUATIONS IN A SEMI-ARID REGION

This paper evaluates three different methods for estimation of reference crop evapotranspiration (ETo) in Bajgah area, Fars province, in Iran. Monthly values of ETo were estimated with Penman-FAO, Penman-Monteith and Hargreaves equations using the mean monthly weather data from 1986-2002 (17 years). The Penman-Monteith and Hargreaves equations were calibrated based on Penman-FAO method which was recognized as the most appropriate equation for ETo estimation, according to the previous studies. This study showed that the results of Penman-Monteith and Hargreaves equations are similar to but somehow they underestimate ETo compared with Penman-FAO method. To calibrate the Penman-Monteith and Hargreaves equations, a linear regression was used. The results of calibration for PenmanMonteith equation showed the variation of the slope of the line and ET0 the calibration coefficient (CH) of the Hargreaves equation for each month. annual ETo estimated.