New cognition on the response of reference evapotranspiration to climate change in China using an independent climatic driver system

Reference evapotranspiration (ETo) is a key parameter in hydrometeorological studies, but dependent climatic drivers can induce uncertainties when attributing changes of ETo to climate change, and many studies have neglected this issue. In this study, two sets of climatic drivers (the independent/dependent driver system) were used to attribute changes of Penman-Monteith ET(o )to climatic variables in China. Results show ETo presented an upward-downward-upward trend with the change points in 1978 and 1996. Using the dependent driver system, the sensitivity coefficient and the contribution of average air temperature (Tmean) were underestimated by at least 57% and 46% during three periods (1960-1978, 1979-1996 and 1997-2019). The largest relative changes of contribution of Tmean (C_Tmean) were found in southeast of China with high temperatures, while largest relative changes of C_Tmean peaked in autumn. Therefore, in order to promote the acquisition of relatively objective results in attributing changes of ETo, the independence of climatic drivers must be carefully addressed. Using the independent driver system, we found ETo was most sensitive to Tmean in spring, summer and winter. Tmean was always a dominant factor for the largest percentage (at least 41%) of grids across China, followed by WS (at most 30%) during three periods. However, the large changes of WS in northwest of China promote the changes of ETo during 1960-1978 and 1979-1996, leading to increased WS, decreased WS and increased Tmean responsible for the upward-downward-upward trend of ETo for entire China as a whole during three periods. This study emphasizes the effect of Tmean in changing ETo during the entire study area, and help improve our understanding of the evolution of ETo and providing a guideline for water resource management and water use planning for agriculture.