## 3 Results  
### Flash Droughts and Associated Meteorological Characteristics  
The correlation coefficients of the daily SM series from 2001 to 2020 were high between SMCI 1.0 and ERA5 (Figure S1, Supplementary Materials), and were greater than 0.7 in most parts of the study area. Thus, the SMCI 1.0 dataset better reflects the SM characteristics for the study area. The spatial patterns of CSIF, GOSIF, and GPP exhibited fundamental similarities (Figure 4). The values were high in the SCR region, with annual means of GOSIF and CSIF basically exceeding 0.2 mW\(\cdot\)m\({}^{-2}\cdot\)nm\({}^{-1}\cdot\)sr\({}^{-1}\), and annual means of GPP exceeding 4 gC\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\). The agricultural regions in the Yangtze River basin (UYR, MYR, and LYR) have annual means of SIF ranging from 0.05 to 0.2 mW\(\cdot\)m\({}^{-2}\cdot\)nm\({}^{-1}\cdot\)sr\({}^{-1}\), and annual means of GPP ranging from 2 to 4 gC\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\). The trends of CSIF, GOSIF, and GPP in the study area were relatively similar (Figure S2), with clear greening trends of vegetation in most of the zones. Figure 4: Spatial patterns of annual averages of (**a**) CSIF, (**b**) GOSIF, and (**c**) GPP for the period of 2001–2020. Figure 3: Flowchart illustrating the methodologies utilized in this study. The total number of flash droughts in the study area was 5-20 events during 2001-2020 (Figure 5a). The total frequency was high (>15 events) in the eastern part of the UYR, the central part of the MYR, and the eastern part of the LYR. We decomposed the total frequency of flash droughts into Case 1, Case 2, and Case 4 events based on the rapidity of drought occurrence. Case 3 events were not analyzed in this study due to very low frequency. Areas with mostly (80%) Case 1 events were mainly in the southern part of the MYR and LYR, and most of the SCR (Figure 5b). Overall, there were fewer Case 2 events than Case 1 events, with proportions mainly in the range of 20-60% in the crop areas of the Yangtze River basin (Figure 5c). Case 4 events accounted for less than 10% in most regions, with a few exceptions of high values in the eastern areas of UYR (Figure 5d). The average durations of flash drought events were determined by dividing the total durations by the total frequency count. As shown in Figure 6a, the mean durations of total flash drought events were mainly concentrated in 6-12 pentads, and their spatial characteristics were almost inversely distributed with the total frequency characteristics. The mean durations of the Case 1 events were more than 10 pentads in most regions (Figure 6b). The spatial durations of Case 2 events were similar to those of the total events, but the specific values were relatively small (Figure 6c). There were almost no Case 4 events in most of the SCR and in the southern parts of the MYR and LYR, so we used white to indicate the Case 4 durations in these regions (Figure 6d). It is interesting to note that for Case 4 events in most of the UYR, as well as in the central and northern areas of MYR and LYR, their mean durations were concentrated in <6 and >12 pentads, suggesting that these events with the highest rapidity of drought have either very short or very long mean durations. Figure 5: Spatial patterns of (**a**) total flash drought frequency and proportions of (**b**) Case 1, (**c**) Case 2, and (**d**) Case 4 events relative to the total events during 2001–2020. We next investigated the area proportions of flash drought per year. The area proportion was defined as the number of grids with at least one flash drought event per year divided by the total count of grids across the study area. As depicted in Figure 7a, the proportion of flash drought-affected areas in the main agricultural areas in the southern parts of China showed a significant decreasing trend (\(-8.23\%\)/decade), consistent with decreasing trends of Case 1, Case 2, and Case 3 events.