## 3 Results
### Flash Droughts and Associated Meteorological Characteristics
Relatively large proportions of area with total flash droughts were recorded in 2001, 2004, 2005, 2006, 2007, 2010, 2013, and 2019, accounting for about half of the total grids. The area proportion changes of Case 1, Case 2, and Case 3 events mirrored those of the total events, yet the specific proportion values were notably lower, especially for the Case 4 events. To further verify the reliability of the SMCI 1.0 in identifying the proportion of area with flash droughts, we compared the area proportions of flash droughts using 0-30 cm SM of SMCI 1.0 and 0-28 cm SM of ERA5. Although the area proportions of flash droughts based on ERA5 data were generally higher than those based on SMCI 1.0 (Figure S3), both data show decreasing trends in the area proportions of flash droughts from 2001 to 2020, with consistent changes. Overall, our analysis indicated that the SMC 1.0 has high reliability in recognizing flash droughts. For the extended study period of 1981-2020 (Figure S4), the area proportion of flash droughts based on ERA5 0-28 cm showed a significant upward trend (\(6.11\%\)/decade), especially for Case 4 events. This suggests high long-term risks of flash droughts in the main agricultural areas of southern China, especially for the most rapidly occurring type of events (i.e., Case 4). The durations of flash droughts (trend = \(-1.09\) pentads/decade, P = 0.123) had a slightly more decreasing trend than the proportion of area during 2001-2020 (Figure 7b). Figure 6: Mean durations of flash droughts for (**a**) total, (**b**) Case 1, (**c**) Case 2, and (**d**) Case 4 events during 2001–2020. The top three years in the last two decades with long flash drought durations were 2019 (7.22 pentads), 2010 (7.06 pentads), and 2013 (6.74 pentads), indicating severe flash droughts in these years. The standardized anomalies of precipitation were basically negative during flash drought conditions (Figure 8a), with more pronounced negative values (around \(-0.3\)) in the central and southern areas of MYR, the northern areas of LYR, and the central areas of SCR. The standardized anomalies of PET were basically positive under flash drought conditions (Figure 8b), with a range of 0.2 to 0.6, especially in the LYR region. The spatial standardized anomalies of P-PET were similar to those of the precipitation and were basically negative (Figure 8c), mainly ranging from \(-0.5\) to \(-0.1\). The spatial standardized  
Figure 7: **(a)** The area (grid) proportions of flash droughts and (**b**) the durations of flash droughts during 2001–2020. anomalies of T\({}_{\text{max}}\) were similar to those of PET (Figure 8d), as expected, since PET was calculated from temperature and latitude information. Under flash drought conditions, the areas with standardized anomalies of VPD greater than 0.6 were significantly larger than the PET (Figure 8e), especially in the LYR and SCR. The standardized anomalies of wind speeds were positive and negative under flash drought conditions in different areas (Figure 8f), with the positive values mainly located in the UYR. The spatial standardized anomalies of these meteorological variables suggest that precipitation deficits and high temperatures tend to lead to enhanced evapotranspiration capacity, which in turn lead to P-PET exhibiting negative anomalies and ultimately tending to induce flash drought events. To gain a clear understanding of the mechanisms behind flash droughs, we examined the temporal evolutions of meteorological variables associated with drought conditions. During flash droughs, we observed a below-normal water quantity, indicated by P and P-PET, as well as an above-normal evapotranspiration capacity, reflected in T\({}_{\text{max}}\), PET, and VPD (Figure 9a). There was little obvious change in wind speed during the evolution of flash drought, suggesting less effect of wind speed on flash drought in the study area. Precipitation and P-PET generally dropped rapidly to their maximum negative values about two pentads earlier than SM.