## 4 Discussion
In this study, the definition of flash droughts incorporated the swift determination of SM towards drought conditions, along with the SM thresholds associated with drought categories, so a sufficiently long period of water deficits adversely affects the ecosystem. The hotspots of flash droughts were located primarily in the eastern parts of the UYR, with the most rapid type (Case 4 events) accounting for a high percentage (about 50%) of the total events in this region. This is primarily due to the anomalously strong western Pacific Subtropical High, a large-scale anticyclonic circulation that significantly influences the weather and climate in East Asia, resulting in heatwaves [88] that can quickly deplete SM and cause flash droughts, resulting in crop damage. Flash droughts with longer average durations occurred in the southern parts of the MYR and LYR, and about half of the SCR. These areas are susceptible to prolonged control by the subtropical high, making flash droughts last longer [20]. The analysis showed an overall decreasing trend in flash droughts in the study area from 2001 to 2020. This conclusion is similar to the findings of previous studies [20,21]. When the study period was extended forward by two decades (1981-2020), flash droughts in the agricultural areas of southern China showed an upward trend. Our results are similar to the findings of previous publications [18,89]. Anthropogenic warming may accelerate hydrological cycle processes and increase the risk of precipitation whiplash events [66,90], which may increase the probability of flash droughts. [PERSON], et al. [91] emphasized the risk of a long-term increase in flash drought frequency may still exist under climate warming in China. El Nino-Southern Oscillation (ENSO) can highly influence precipitation volatility [90], which governs the long-term changes in precipitation, PET, and VPD in southern China, which are crucial factors in the occurrence of flash droughts. An elevated frequency of El Nino can increase the risk of flash drought co-concurrence in global major croplands [49]. The influence of ENSO on flash droughts in major agricultural regions of southern China under different emission scenarios in the future will require further in-depth investigation. During the SM-derived flash drought, P and P-PET showed rapid downward trends prior to the SM. T\({}_{\text{max}}\), PET, and VDP were similarly variations, with roughly opposite changes to the SM under SM-derived flash drought. These results demonstrate that it is feasible to use simple water balance (P-PET) and evaporative stress ratio (ESR) to identify flash droughts [18; 92; 93]. Both historical and future high-resolution SM data are difficult to obtain, especially in soils with deep crop roots (about 50 cm depth). Both P-PET and ESR can identify flash droughts, but the actual evapotranspiration in ESR is more difficult to obtain and varies widely between algorithms. Surface SM and root zone SM lagged behind P-PET by 1-2 pentads and 2-3 pentads, respectively, under conditions of P-PET-derived flash droughts [18]. This suggests the feasibility of early warning for flash droughts stemming from SM depletion. P-PET-derived flash droughts are capable of capturing the rapid drought intensification that leads to SM depletion and drought impacts. Therefore, future work should examine P-PET-derived flash drought impacts on crop SIF, especially for different emission scenarios. Vegetation relies mainly on SM for water uptake, affecting transpiration, stomatal conductance, and stem-water dynamics [8; 94]. Rapid declines in SM can reduce stomatal conductance to prevent additional water loss [95]. At the same time, the rate of carbon dioxide diffusion into the plant leaves can decrease, resulting in weakened overall plant photosynthesis capacity with a decrease in SIF. The average response frequency of SIF in major agricultural areas in southern China was above 80% during flash drought, and the average response time was within two pentads.