In this study,seven widely used potential evapotranspiration(ETo)methods were evaluated by comparing with the FAO-56 Penman-Monteith method(PM method)to provide useful information for selecting appropriate ETo equations under data-limited condition in Beijing,China.Statistical methods and parameters,namely linear regression,root mean squared error(RMSE)and mean bias error(MBE),were used to evaluate the seven ETo methods.Results showed that ETo estimated using Kimberly-Penman method have fairly close agreement with the PM method(referring to standard ETo),considering the coefficient of determination(R^(2))of 0.96,RMSE of 0.42 mm/day,and a coefficient of efficiency(E)of 0.96.Locally calibrated Penman and Doorenbos-Pruitt methods also have better agreement with the PM method,correspondingly with R^(2)of 0.99 and 0.95,RMSEs of 0.24 mm/day and 0.21 mm/day,and coefficients of efficiency of 1.02 and 0.99,respectively.The ETo is the most sensitive to vapor pressure deficit(VPD)and net radiation in the Beijing area.Hence,the VPD-based and VPD-radiation combined ETo methods were developed and calibrated.Results showed that the two developed methods performed well in ETo estimation.By fully considering the data-limit situation,the calibrated Turc method,VPD-based method and VPD-radiation-combined method may be attractive alternatives to the more complex Penman−Monteith method in Beijing.
Linhe National Meteorological Station, a representative weather station in the Hetao Irrigation District of China, was selected as the research site for the present study. Changes in climatic variables and reference evapotranspiration （ET0 ） （estimated by the Penman-Monteith method） were detected using Mann-Kendall tests and Sen＇s slope estimator, respectively. The authors analyzed the relationship between the ET0 change and each climatic variable＇s change. From 1954 to 2012, the air temperature showed a significant increasing trend, whereas relative humidity and wind speed decreased dramatically. These changes resulted in a slight increase in ETo. The radiative component of total ET0 increased from 50% to 57%, indicating that this component made a greater contribution to the increase in total ETo than the aerodynamic component, especially during the crop growing season （from April to October）. The sensitivity analysis showed that ETo in Hetao is most sensitive to mean daily air temperature （11.8%）, followed by wind speed （-7.3%） and relative humidity （4.8%）. Changes in sunshine duration had only a minor effect on ET0 over the past 59 years.
