## 1 Introduction
used GLEAM ET data to analyze the changes and dynamic characteristics of actual ET and its components in the YRB from 1980 to 2014 and found that the actual ET showed a significant increasing trend, especially in the middle and lower reaches of the YRB [22]. Therefore, it was not difficult to speculate that the overall actual ET in the YRB from 1960 to 2014 showed a trend of first decreasing and then increasing in a fluctuating pattern, but the change patterns varied from region to region, which was similar to the change trend of ET\({}_{0}\) in the YRB. However, most existing studies used station-based actual ET measurement and ET actual data with low spatial resolution to investigate the spatiotemporal characteristics of ET in the YRB with high spatial heterogeneity, which may lead to some uncertainties. The understanding of ET change trend in the YRB in recent years has been lacking. Several studies have shown that ET in China is mainly influenced by meteorological elements such as wind speed, air humidity, precipitation, temperature, and sunshine duration; however, the vast territory and complex topography of China lead to different climatic influences on ET in different regions [29; 30; 31; 32]. Further, the response of ET to climatic factors in the YRB has been explored in several studies. [PERSON] et al. analyzed the sensitivity of ET\({}_{0}\) to key climate variables in the YRB during 1960\(-\)2000 and found that relative humidity was the most sensitive variable, followed by short wave radiation, air temperature, and wind speed, and the ranking of sensitivity of the four climatic variables varied with seasons and regions [33]. [PERSON] et al. found that relative humidity, wind speed, and sunshine duration were the main meteorological variables affecting annual ET\({}_{0}\) changes in the Yangtze River Delta during 1957\(-\)2014 [25]. [PERSON] et al. found that the spatial pattern of actual ET in the YRB during 1980-2014 was jointly determined by air temperature and precipitation, while solar radiation was the dominant controlling factor of actual ET in most regions [22]. [PERSON] et al. quantified the influence of climatic variables on ET\({}_{0}\) in the UYRB from 1951 to 2020 and found that relative humidity was the main factor affecting ET\({}_{0}\) changes, and the contribution of climatic variables to ET\({}_{0}\) changes in each sub-basinwas different [19]. Combining previous studies, it is found that the ET variation in the YRB is highly influenced by climatic and geomorphological factors such as air temperature, precipitation, wind speed, solar radiation, and specific humidity. However, the effects of climatic factors on ET variation are only explored at the annual scale in the existing studies. The YRB has a typical monsoon climate with four distinct seasons, and climatic factors have obvious seasonal variation characteristics [17]; thus, the response of ET driven by climatic factors varies with the seasons. In order to explore the actual ET changes and their driving mechanisms in the YRB in the last two decades, this study quantitatively analyzed the spatiotemporal characteristics of annual and monthly ET using the Theil--Sen median trend analysis and the Mann--Kendall test based on the ET dataset of the PML_V2 model from 2001 to 2020. In addition, combined with air temperature, solar radiation, precipitation, specific humidity, and wind speed time series data, this study further explored the effect of climate change on ET at different spatial and temporal levels using correlation analysis, normalized multiple linear regression analysis, and other methods to provide scientific reference for ecological protection, rational development, and utilization of water resources and climate change in the YRB. This study aims to answer: (1) What are the temporal and spatial characteristics of annual and monthly ET changes in the YRB during 2001\(-\)2020? (2) What are the driving mechanisms of climatic factors affecting ET change in the YRB at the annual and monthly scale?