## 2 Materials and Methods
### Ability of SIF to Monitor Drought-Induced Changes in NPP Dynamics
The monthly comprehensive drought index for the HHH Plain reveals that the two prolonged droughts included periods of mild, moderate, severe, and extreme drought conditions (apart from the initial stage of the 2019 drought, which did not exhibit extreme drought conditions). Consequently, we extracted the regions where different drought events occurred within the HHH Plain and calculated the average standard anomaly of GOSIF and GLASS NPP values in these regions. Subsequently, we constructed SIF/NPP time series with an 8-day interval to analyze the variations in SIF/NPP ratios during different drought events. Our aim is to explore the potential of SIF for monitoring dynamic changes in NPP under drought conditions of differing severity. During October 2013 and September 2014, the SIF/NPP ratio significantly changed during a mild drought event (Figure 8a). At the beginning of October, the ratio quickly increased from approximately \(-10\) to approximately \(6\) before fluctuating and gradually stabilizing. During this period, the SIF/NPP ratio was relatively stable and a synchronous trend in change was present between SIF and NPP. In April 2014, the SIF/NPP ratio exhibited significant fluctuations for approximately \(3.5\) months, dropping rapidly from approximately \(7\) to \(-4\). During this time, the relationship between SIF and NPP changed from synchronous to asynchronous, and this state of asynchronous change lasted for approximately one month. In mid-May, the SIF/NPP ratio underwent another substantial fluctuation and the relationship between SIF and NPP briefly changed from asynchronous to synchronous. From mid-July until the end of the drought, the SIF/NPP ratio gradually changed from negative to positive and then stabilized. The relationship between SIF and NPP became synchronous and gradually stabilized. The changes in the SIF/NPP ratio were similar in general and during severe drought events (Figure 8b,c). At the beginning of the drought, the SIF/NPP ratio was generally positive and exhibited fluctuations followed by stability, with a relatively stable relationship between SIF and NPP. From early April 2014 until July, the SIF/NPP ratio fluctuated significantly, similar to that observed during the mild drought event, and the dynamic relationship between SIF and NPP became asynchronous. From mid-July until the end of the drought, the SIF/NPP ratio gradually returned to positive values and stabilized thereafter. The relationship between SIF and NPP gradually became synchronous and the two became increasingly stable. In extremely severe drought events (Figure 8d), the trend in SIF/NPP ratio changes was similar to that observed in the other three types of droughts. However, overall, the SIF/NPP ratio exhibited more significant fluctuations, whereas in the later stages of drought, the SIF/NPP ratio was mostly negative, indicating asynchronous dynamic changes between SIF and NPP. These findings suggest that the relationship between SIF and NPP is unstable during extremely severe drought events and that the ability of SIF to monitor the dynamic changes in NPP induced by drought is weaker than its ability during other levels of drought. Between May and December 2019, during a mild drought event (Figure 9a), the SIF/NPP ratio was negative from May to mid-July, indicating asynchronous changes between SIF and NPP. In approximately late July, the SIF/NPP ratio transitioned to positive values and gradually stabilized, demonstrating synchronous dynamics between SIF and NPP with a relatively stable relationship. From mid-September to mid-October, the SIF/NPP ratio underwent drastic fluctuation, dropping rapidly to approximately \(-10\) before rising to approximately 17. During this period, the relationship between the SIF and NPP became asynchronous. From mid- to late-October until the end of the drought, the SIF/NPP ratio gradually declined, approaching the multi-year average ratio. At this time, the dynamic changes in SIF and NPP were similar and their relationship gradually stabilized.