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(ET_0)(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 ET_0 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 ET_0. The radiative component of total ET_0 increased from 50% to 57%, indicating that this component made a greater contribution to the increase in total ET_0 than the aerodynamic component, especially during the crop growing season(from April to October). The sensitivity analysis showed that ET_0 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 ET_0 over the past 59 years.
