| Summary: | Evapotranspiration is a key process between the atmospheric hydrological cycle and the energy cycle, which has a great significance in understanding climate change and the rational use of water resources, especially for the Weihe River basin (WRB) (a basin in China experiencing a shortage of water resources). We investigated the spatial-temporal change of actual evapotranspiration (<i>ET</i><sub>a</sub>) based on the daily meteorological variables of 22 meteorological stations and the annual streamflow of three hydrological stations from 1970 to 2018 in the WRB. The contributions of key meteorological variables to <i>ET</i><sub>a</sub> changes and the sensitivity coefficient are also quantified. The temporal trends of <i>ET</i><sub>a</sub> showed an increasing trend from 1970 to 2018, and the spatial distribution of <i>ET</i><sub>a</sub> increased from northwest to southeast in the WRB. Increasing trends were detected in the multi-year average, spring, and winter, but only a few stations passed the significance test. Summer and autumn showed a decreasing trend, but this trend was not significant. Solar radiation is the most sensitive meteorological variable, followed by vapor pressure, wind speed, and mean temperature. Vapor pressure contributes the most to <i>ET</i><sub>a</sub> changes, followed by solar radiation. In general, vapor pressure (relative humidity) is the dominant meteorological factor affecting <i>ET</i><sub>a</sub> in the WRB. In addition to meteorological factors, the <i>ET</i><sub>a</sub> is also affected by combined and complicated factors caused by precipitation and human activities. As an important part of the hydrological cycle, <i>ET</i><sub>a</sub> has important research significance for water resources management, economy, agriculture, and ecology and results of this study may be helpful to further clarify the climate change and human activities impacts on the basin hydrological cycle.
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