## 3 Results
### Typical Case of 2019 Flash Drought  
The impact area was extensive and there was significant duration of flash drought in 2019, so this event was selected for further analysis. As shown in Figure 13a, MYR and LYR were more widely affected by flash drought in 2019. On the grids where flash drought  
Figure 12: Temporal evolution of CSIF, GOSIF, and GPP during flash droughts for (**a**) total, (**b**) Case 1, (**c**) Case 2, and (**d**) Case 4 events in the study area 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 vegetation indices. 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 \(n^{\prime\prime}\).  
occurred, about 50% of the grids had the start of a flash drought in the 41 st-47 th pentads (i.e., 20 July-23 August) of 2019 (Figure 13b). The largest proportion of grids experiencing flash droughts started at the 42 nd pentad of 2019, and this mainly occurred in the southern parts of the MYR and LYR.  
We investigated the temporal changes of meteorological variables and vegetation photosynthesis capacity during a typical flash drought event based on all grids where the flash drought event started at the 42 nd pentad of 2019. As shown in Figure 14a, T\({}_{\text{max}}\) and VPD rose dramatically in the 1-3 pentads preceding the onset of flash drought. These conditions led to a substantial enhancement in evapotranspiration capacity, ultimately resulting in a sharp decline in P-PET and consequently a rapid depletion of SM. T\({}_{\text{max}}\) and VPD exhibited largely positive anomalies during the 42 nd-57 th pentads of 2019, especially at the 51 st and 55 th pentads (standardized anomaly as high as nearly 2). Standardized anomaly of P-PET was at nearly \(-1\) for much of the 42 nd-57 th pentads of 2019. For specific values of meteorological variables, P-PET ranged between \(-2\) and \(-5\) mm for most of the 40 th-62 nd pentads of 2019 (Figure S11a). Despite a small increase in P-PET at the 60 th pentad, which proceeded to cause a slight increase in SM, it was difficult to return to a normal SM state. T\({}_{\text{max}}\) was essentially above 32 \({}^{\circ}\)C during the 40 th-55 th pentads of 2019 (Figure S11b), especially the six pentads following the onset of flash drought (reached about 35 \({}^{\circ}\)C). The sustained high temperatures resulted in a substantial increase in VPD, with approximately five pentads of VPD as high as 1.5 kPa during this flash drought (Figure S11c). The prolonged high temperatures and high evapotranspiration capacity, as well as the shortage of atmospheric moisture, led to a prolonged period of low SM, which in turn increased the severity of this flash drought.  
Photosynthetic capacity of vegetation generally reached negative anomalies with a lag of roughly 3-5 pentads in response to the 2019 flash drought (Figure 14b). Standardized anomalies of CSIF, GOSIF, and GPP were largely negative during POY 46-58 of 2019, peaking at negative anomalies (below \(-0.5\)) at the POY 53. Overall, CSIF showed less variability than the GOSIF and GPP, but was more sensitive to drought conditions than both the GOSIF and GPP. Around POY 60, a slight increase in precipitation resulted in improved growth (elevated standardized anomalies), but specific vegetation photosynthetic capacity remained at low levels (Figure S11d-f). Overall, 2019 flash drought events severely inhibited the growth of crops in the study area, resulting in a huge grain yield reduction.  
Figure 13: Spatial distributions of (**a**) the flash drought frequency in 2019 and (**b**) the start dates of flash droughts during 41 st–47 th pentads (20 July–23 August) of 2019. POY: pentad of year. The total number of flash drought events at the grid scale is labeled as “\(n\)”.