## 2 Materials and Methods
### Ability of SIF to Monitor Drought-Induced Changes in NPP Dynamics
2.1 Analysis of the Relationship between SIF and NPP in Different Drought Events Based on SIF/NPP Variations  
During the drought events from October 2013 to September 2014 (Figure 7a), the SIF/NPP ratio was positive at the beginning of the drought and gradually decreased with a fluctuating trend. By the end of November 2013, the SIF/NPP ratio had decreased from 5 to approach the ratio\({}_{\text{mean}}\) of 1.5189, and remained proximate to this value from December 2013 to February 2014. During the later stages of drought, the SIF/NPP ratio fluctuated substantially. From March to mid-April 2014, the SIF/NPP ratio changed drastically, forming two intense peaks. At the end of March, the SIF/NPP ratio had a small peak close to 10, which dropped rapidly to approximately \(-10\). In mid-May, the SIF/NPP ratio had another peak of approximately 10, which rapidly decreased to approximately \(-5\). During the later stages of the drought, the SIF/NPP ratio gradually became positive. From mid-June 2014 to the end of the drought, SIF and NPP showed synchronous changes and their relationships were stable. During the initial stage of the drought event, from May to December 2019 (Figure 7b), the SIF/NPP ratio fluctuated drastically for approximately three months. Near the end of May, the SIF/NPP ratio reached an extremely low value of approximately \(-16\), then rapidly increased to approximately 13, and subsequently decreased to approximately \(-15\). During this period, the relationship between SIF and NPP showed overall instability. From the end of July, the SIF/NPP ratio gradually returned to positive values and approached stability, with a trend close to the multi-year average SIF/NPP. At this point, the relationship between SIF and NPP changed from asynchronous to synchronous and gradually stabilized. Beginning in mid-September, the SIF/NPP ratio fluctuated again, with the ratio first dropping to approximately \(-5\) and then rapidly rising to approximately 15. At this point, the relationship between SIF and NPP was unstable. From mid-October, the SIF/NPP ratio gradually stabilized and approached the multi-year average. During this time, the SIF and NPP trends were synchronous, and the relationship between the two was relatively stable. In both drought events, the SIF/NPP time series changes exhibited clear seasonality and showed similar characteristics at the same time points. During the late spring, spring-summer, and summer-autumn transitions, the dynamic changes in NPP and SIF were not synchronized, and the SIF/NPP ratio showed large fluctuations. At this time, the dynamic changes in SIF and NPP were often not synchronized, and SIF could not accurately capture the dynamic changes in NPP induced by drought. In early spring, summer, autumn, and winter, the dynamic changes in SIF and NPP were synchronized. The SIF/NPP index  
Figure 7: Dynamic changes in SIF/NPP with 8-day time intervals from October 2013 to September 2014 (**a**) and May to December 2019 (**b**). in autumn and winter was close to the SIF/NPP multi-year average value. In summer, the dynamic change in NPP was greater than that in SIF, resulting in an SIF/NPP value smaller than the SIF/NPP multi-year average value. Overall, SIF and NPP maintained a stable linear relationship in summer, autumn, and winter; thus, SIF accurately monitored dynamic changes in NPP induced by drought. 2.2 Analysis of the Relationship between SIF and NPP under Different Drought Levels Based on SIF/NPP Ratio  
In our previous study [60], run theory was mainly used to identify drought events. This method identifies and quantifies drought events from a global perspective. However, during droughts, the degree of drought is spatially heterogeneous owing to various natural and artificial factors. The drought events identified by run theory neglected the different degrees of drought in the local areas of the HHH Plain.