## 3 Results
### Performances of SIF in Grassland GPP Monitoring under Drought Conditions  
To visually demonstrate the ability of SIF to monitor GPP under drought conditions, we compared and analyzed the characteristics of GPP, SIF, SIF\({}_{\text{yield}}\), and NDVI during the 2009 drought. Figure 8 shows the seasonal variations in GPP, SIF, SIF\({}_{\text{yield}}\), and NDVI in 2009 compared to the multi-year averages. The multi-year averages and the values for 2009 exhibited similar changing patterns, with an initial increase followed by a decrease. In July 2009, compared to the multi-year averages, GPP, SIF, and NDVI declined 26.18%, 12.43%, and 16.26%, respectively. In August, GPP, SIF, and NDVI declined 30.33%, 19.89%, and 19.65% compared to the multi-year averages, respectively (Figure 8). However, the difference between SIF\({}_{\text{yield}}\) in 2009 and the multi-year average was relatively small. During a drought event, reduced rainfall leads to soil moisture deficit, ultimately affecting vegetation growth. In 2009, GPP reached its peak in June and then started to decline, while SIF, SIF\({}_{\text{yield}}\), and NDVI continued to increase until July before decreasing. This indicated a lag in the response of SIF and NDVI to vegetation growth changes. All  
Figure 8: Seasonal variations of (**a**) GPP, (**b**) SIF, (**c**) SIF\({}_{\text{yield}}\), and (**d**) NDVI in the Xilingol grassland from April to October. The red point represents the average monthly values for all pixels of the study area in 2009, while the black point indicates the monthly average for all pixels of the study area from 2007 to 2015. The red curve represents the variations in each parameter from April to October 2009, while the black curve indicates the monthly average from 2007 to 2015. The gray shading represents the standard deviation of the monthly average values from 2007 to 2015. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Vegetation Indicators** & **SPEI** & **SM** & **PPT** & **LST** \\ \hline GPP & 0.42 ** & 0.46 ** & 0.91 *** & 0.73 *** \\ SIF & 0.36 & 0.41 ** & 0.91 *** & 0.71 *** \\ SIF\({}_{\text{yield}}\) & 0.36 & 0.28 & 0.82 *** & 0.66 *** \\ NDVI & 0.32 & 0.53 ** & 0.85 *** & 0.64 *** \\ \hline \hline \end{tabular}  
*** Significance at the 0.001; ** Significance at the 0.01. Linear relationships between drought indices (SPEI, SM, PPT, and LST) and vegetation indices (GPP, SIF, SIF\({}_{\text{yield}}\), and NDVI) with the monthly average value for the entire study area. Each point represents a monthly average value for the entire study area. There are 12 (months/year) \(\times\)9 (years) = 108 points from 2007 to 2015.  
\end{table} 
Table 3: Correlation coefficients (r) between drought indices (SPEI, SM, PPT, and LST) and vegetation indices (GPP, SIF, SIF\({}_{\text{yield}}\), and NDVI). indices declined not only during 2009 but in the multi-year data after July. However, after July, both GPP and SIF rapidly decreased due to worsening drought conditions, while NDVI exhibited a slower decline. As can be seen from Figure 8, GPP decreased by 19.57% and 24.33% in August and September 2009, respectively. Similarly, SIF decreased by 27.87% and 54.03% in August and September, respectively. However, NDVI decreased by only 4.05% and 7.92% in August and September, respectively. SIF, although it did not capture the downward trend of GPP in July, decreased by a similar amount in August and September. This also indirectly proves that SIF is better than NDVI in monitoring the changes of GPP. In order to reveal the relationship between SIF, SIF\({}_{\text{yield}}\), NDVI, and GPP more intuitively, it is necessary to carry out a correlation analysis of SIF, SIF\({}_{\text{yield}}\), NDVI, and GPP. Subsequently, we analyzed the correlations between SIF, SIF\({}_{\text{yield}}\), NDVI, and GPP separately for all drought and non-drought months. Figure 9 revealed that drought reduced the ability of NDVI to monitor GPP, with R\({}^{2}\) values of 0.86 (no_drought) and 0.62 (drought), but drought did not affect that of SIF to monitor GPP.