Comparison and modification of equations for calculating evapotranspiration (ET) with data from Gansu Province, Northwest China

A total of 11 empirical evapotranspiration equations were developed by Hargreaves (1975), Hargreaves (1985), Makkink, Turc, Priestley-Taylor, Jensen-Haise, Doorenbos and Pruitt, Abtew, McGuinness and Bordne, Rohwer and Blaney-Criddle. These were compared with pan evaporation data measured at 10 stations in Gansu Province, northwest China. The comparison was first made using the default values in each of the 11 equations, and then redone using calibrated coefficient values. The calibration was made for all 11 equations using the climatic index Pa/Pg. The results show that using default coefficient values resulted in large errors for all equations. The largest error was found in the comparison results of the Turc, and McGuinness and Bordne equations. The Turc and Jensen-Haise equations should be used to estimate the daily evapotranspiration (ET) under the conditions of Tmean > 0 and Tmean > -3°C, respectively, when applied to northwest China. Based on the comparison results of before and after recalibration, the Doorenbos and Pruitt method is more suitable to the arid, semiarid and wet regions in northwest China. The accuracy of the results for determining monthly ET greatly improved when calibrated coefficient values were introduced to the 11 equations, except for McGuinness and Bordne, Abtew, and Blaney-Criddle, which need to be recalibrated using two parameters. Thus, it can be concluded that calibration can be used to modify evapotranspiration equations with multi-station data to improve the accuracy of evapotranspiration estimates in northwest China. Copyright © 2009 John Wiley & Sons, Ltd.