## 4 Discussion  
Serious precipitation deficits and high evapotranspiration capacity force a rapid decline in SM, triggering flash droughts, which in turn cause rapid declines in the photosynthetic capacity of crops (Figure 15). Flash droughts can occur within a few weeks, limiting the time available for early warning of droughts and the effective implementation of mitigation measures [12; 20; 76]. Previous studies attempted to estimate the reduction in GPP or SIF during flash droughts at the national scale or basin scale [8; 37]. However, there was little investigation of the dynamic characteristics of crop photosynthesis during flash droughts, especially in agricultural areas with humid climate attributes. For this reason, we focused on the characteristics of SM-derived flash droughts in the agricultural areas of southern China in the past two decades and assessed the impacts of these events on the photosynthetic capacity of crops via high-resolution SM and SIF datasets. Figure 14: Temporal features of (a) meteorological and (b) vegetation indices during 2019 flash droughts in the study area. In each subplot, the left \(y\)-axis denotes the SM percentile, whereas the right \(y\)-axis denotes the standardized anomalies of the meteorological or vegetation indices. Lines indicate medians and shading indicates the interquartile range. A total of 727 grids had a flash drought start in the 42 nd pentad of 2019. POY: pentad of year. Figure 15: Schematic diagram illustrating the mechanisms of flash droughts and effects on crops. The SIF values showed upward trends in the study area during 2001-2020, indicating that the crops in these regions grew better in the last two decades, allowing increased agricultural production. This finding of increased grain yields is similar to that of [PERSON], et al. [77], who analyzed China's county-level grain production and found that the counties with faster average annual growth rates are mainly located in the Northeast China Plain, Huang-Huai-Hai Plain, and Middle-Lower Yangtze Plain regions. [PERSON] and [PERSON] [78] found that croplands and forests made the largest contributions to vegetation greening in India during 2000-2019, a finding that is similar to the greening of agricultural areas in southern China. Despite China encompassing only 6.6% of the world's vegetated area, it contributes a notable 25% to the global expansion of green leafy areas. Forests and croplands contributed 42% and 32%, respectively, to the greening of vegetation in China [79]. The enhancement of vegetative greenness in agricultural regions of southern China may be related to the increase in temperature and P-PET. In dry and hot environments, water is the main limiting factor for plant growth and photosynthesis, and increased carbon dioxide (CO\({}_{2}\)) concentration can increase the efficiency of water use by vegetation (CO\({}_{2}\) fertilization effects). Therefore, elevated atmospheric CO\({}_{2}\) concentration is an important driver of vegetation greening [80,81]. However, under compound heatwaves and droughts, even if the atmospheric CO\({}_{2}\) concentration increases, the stomatal conductance of the vegetation decreases due to anomalous water scarcity, thus inhibiting vegetation photosynthesis [82]. Under climate warming, there are distinctly upward trends observed in hot and dry extremes, including compound heatwaves and droughts [83,84]. The increases in these extreme events favor the occurrence of flash droughts, which are predicted to continue to rise in the future [18,85]. Enhanced compound hot and dry events over agricultural areas (croplands) can increase risks of crop damage in adjacent areas [86,87]. Simultaneous occurrence of flash droughts increased in global major croplands under climate warming [49]. However, there is a lack of quantitative studies on the impacts of flash drought co-occurrence on the photosynthetic capacity of vegetation in agricultural areas, especially under future high-emission scenarios.