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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 ET0. The radiative component of total ET0 increased from 50% to 57%, indicating that this component made a greater contribution to the increase in total ET0 than the aerodynamic component, especially during the crop growing season (from April to October). The sensitivity analysis showed that ET0 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.