## 3 Results
### Response of Vegetation Photosynthesis to Flash Droughts  
The spatial distributions of response frequencies of CSIF (Figure 10a), GOSIF (Figure 10b), and GPP (Figure 10c) during flash drought were characterized consistently, and more than half of the grids in the total region had response frequencies above 80%. The response frequency of vegetation photosynthesis capacity reached 90% in the western areas of UYR, the northern and southern areas of LYR, the southern and eastern areas of LYR, and the southwestern and eastern areas of SCR. This result indicates that flash drought can seriously alter crop photosynthesis. The response frequency of vegetation photosynthetic capacity to Case 1, Case 2, and Case 4 events was around 80% for most grids (Figure S5). Although few grids had Case 4 events, more than half of the grids had a response frequency of more than 90%, indicating that the most rapid drought events exhibit more pronounced vegetation response. The spatial distributions of the response time of CSIF (Figure 10d), GOSIF (Figure 10e), and GPP (Figure 10f) to flash droughts were also basically consistent, at generally less than three pentads. Vegetation photosynthesis capacity in most parts of the UYR, the north-central parts of the MYR, the northern regions of the LYR, and most regions of the SCR showed a negative anomaly within one pentad after the onset of flash droughts. This finding suggested that flash droughts in southern China's agricultural regions can rapidly lead to a decrease in the photosynthetic capacity of crops, thus threatening grain yield. Overall, the response time becomes shorter with an increasing onset rate of flash drought (Figure S6). Figure 9: Temporal features of meteorological variables during flash drought conditions for (**a**) total, (**b**) Case 1, (**c**) Case 2, and (**d**) Case 4 events in the main agricultural areas of southern China during 2001–2020. In each subplot, the left \(y\)-axis represents the SM percentile, whereas the right \(y\)-axis denotes the standardized anomalies of the meteorological variables. Lines indicate the median values, and shading denotes the interquartile range. Additionally, the total number of flash drought events at the grid scale is labeled as “\(n\)”. CSIF, GOSIF, and GPP showed negative anomalies during the flash droughts in most of the study area (Figure 11). The areas with standardized anomalies of the photosynthetic capacity \(<-0.5\) were primarily concentrated in the northern areas of the MYR and the southwestern and southeastern parts of the SCR, indicating serious effects on the crops in these areas during flash droughts. According to the temporal evolutions of vegetation photosynthetic capacity during flash droughts (Figure 12), there were basically similar temporal changes of CSIF, GOSIF, and GPP. Prior to the onset of flash drought, the standardized anomalies of vegetation photosynthesis capacity were close to 0 (normal state). When flash drought occurred, SM decreased rapidly, with slower decreases in SIF and GPP than for SM. This reflects a lag effect in the response of vegetation to SM, indicating that vegetation has some drought resistance to low SM. For all flash drought events (Figure 12a), SM reached its lowest value at the third pentad of the flash drought period, and CSIF, GOSIF, and GPP reached their lowest values at about the fourth, seventh, and seventh pentads, respectively. This indicated that CSIF was more sensitive to SM than GOSIF and GPP. For Case 1 events  
Figure 11: Mean standardized anomalies of (**a**) CSIF, (**b**) GOSIF, and (**c**) GPP during flash droughts in the study area during 2001–2020. Figure 10: (**a–c**) Response frequency and (**d–f**) response time of CSIF, GOSIF, and GPP to flash droughts in the main agricultural areas of southern China during 2001–2020. (Figure 12b), SM reached its lowest value at the fourth pentad during the flash drought, and CSIF, GOSIF, and GPP reached their lowest values at about the fifth, seventh, and seventh pentads, respectively.