Comparison and Modification of Evapotranspiration Estimation Method in Upper Hanjiang Basin
汉江上游流域蒸散量计算方法的比较及改进

Five different evapotranspiration (ET0) estimation methods, namely: Penman-Monteith (PM), Pristley-Taylor (PT), Hargreaves, FAO Penman-Monteith (FAO PM) and De Bruin, were used to evaluate the validity and precision of simulated ET0 results with the observed meteorological data from 13 weather stations in upper Hanjiang basin. The objective of this article is to find the best method among them and made further improvement on it to get a higher simulation precision on ET0 simulation in study area. Based on four analysis index study such as correlation coefficient, explanation variance (EV), Nash-Sutcliffe Criterion (NSC) and correlation curve slope for the simulated results and the real ones calculated by 20-cm evaporating dish values with evaporation pan coefficient, PM and FAO PM methods were thought of better methods to yield rather satisfied estimation among the five selected approaches for and FAO PM methods considered both energy balance and water vapor diffusive theory in ET0 simulation model construction while other three methods only considered latter elements. But the results also showed that PM method was better in correlation coefficient and EV indexes while FAO PM method was better in NSC and correlation curve slope indexes. In order to find the most favorable method among them, detailed studies were done and the results showed that FAO PM method has some shortcomings in calculation of the middle-process parameters. For example the simulated saturated water vapor in FAO PM method was lower than observed real water vapor value and the calculated solar radiation was larger than observed one. These will bring errors for some sub-processes simulation in hydrological studies. Therefore, PM method was recommended as the best ET0 estimation approach in upper Hanjiang basin. The final precision of ET0