Abstract: The Han River (Hanjiang) is the largest tributary of the Yangtze River and located in the geographical transitional zone between the north and the south of China. The climate in the basin is mild and humid, with an uneven spatial and temporal distribution of water resources. Evapotranspiration, as an important part of the hydrologic cycle, needs to be comprehensively analyzed to rationally utilize water resources. In this paper, the FAO56 PenmanMonteith equation and four temperaturebased equations were used to estimate the daily and monthly potential evapotranspiration in the Han River basin. The FAO56 PenmanMonteith equation is a synthetic method, which has been recommended by FAO and widely used all over the world. The four temperaturebased methods used in this paper include Hargreaves，BlaneyCriddle，Thornthwaite, and Hamon. These four methods require less meteorological observation data than the FAO56 PenmanMonteith equation，so they are simple in calculation and flexible in application. But each temperaturebased equation has an empirical coefficient to be calibrated, which limits their application across different conditions. The estimation of potential evapotranspiration was based on the daily meteorological observations from 14 gauging stations in Han River basin during the period from 1960 to 2009, while the areal mean potential evapotranspirations of the basin were calculated using the weighting method of inverse distance. With the results of FAO56 PenmanMonteith equation taken as a reference, the results of the four temperaturebased equations using original empirical coefficients were assessed. It has been found that, compared to FAO56 PenmanMonteith equation, large biases existed for the four temperaturebased equations when original coefficients were used. The maximum relative deviation was up to 41%; but for each decade of the study period, the mean annual potential evapotranspirations of temperaturebased equations were linearly related to the FAO56 PenmanMonteith’s results based on the Ftest. The value of the empirical coefficient of each temperaturebased equation was then calibrated to make its results of potential evapotranspiration estimates best approximate those of FAO56 PenmanMonteith. By using the calibrated coefficients，it was found that the relative deviation of the potential evapotranspiration estimates of each temperaturebased equation from those of FAO56 PenmanMonteith reduced significantly, and the maximum discrepancy was less than 0.1%. The simulation of all the four temperaturebased models improved, with similar variation trend and consistent peaks and troughs. The correlation between Hargreaves equation and FAO56 PenmanMonteith equation is the best, with the correlation coefficient of 0.94, and then the second and third are the Hamon and Thornthwaite equation respectively. The last one is BlaneyCriddle equation with a correlation coefficient of 0.76. Based on these analyses, the recommended temperaturebased method for estimating potential evapotranspirations is Hargreaves equation. This is the first time to compare the temperaturebased methods with the FAO56 PenmanMonteith in terms of potential evapotranspirations caculation in Han River basin, which should be useful for research on potential evapotranspiration methods.

Key words: potential evapotranspiration, Hanjiang Basin, temperaturebased models, FAO56 PenmanMonteith