## 6 Conclusions  
Remote sensing vegetation indices and meteorological drought indices have been used to monitor drought for decades. Solar-induced chlorophyll fluorescence provides an alternative observation of terrestrial plants from space, and could monitor agricultural drought more quickly and effectively. In this study, we first examined the response of SIF to drought by analyzing the relationships between GOME-2 SIF and several drought indices, including one-, two-, and three-month SPIs, and the PDSI. We found significant positive correlations between GOME-2 SIF and all drought indices, indicating that SIF responds strongly and quickly to drought. SIF was more sensitive to short-term precipitation and less sensitive to long-term precipitation than VIs (NDVI and NDWI). We compared the seasonal and inter-annual patterns of GOME-2 SIF and flux-estimated GPP, and found good consistency between SIF and GPP in both humid and drought years. The SIF and GPP declined synchronously in drought years \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Correlation** & \multicolumn{3}{c}{**June**} & \multicolumn{3}{c}{**July**} & \multicolumn{3}{c}{**August**} \\ \cline{2-10}
**Coefficients** & **SPI-1** & **SPI-2** & **SPI-3** & **SPI-1** & **SPI-2** & **SPI-3** & **SPI-1** & **SPI-2** & **SPI-3** \\ \hline
1401 \& 1404 & 0.682 & 0.736 & 0.928 & 0.687 & 0.865 & 0.862 & 0.477 & 0.699 & 0.807 \\
1406 \& 1409 & 0.690 & 0.710 & 0.695 & 0.805 & 0.888 & 0.867 & 0.450 & 0.814 & 0.898 \\
2501 \& 2502 & 0.622 & 0.724 & 0.781 & 0.687 & 0.659 & 0.762 & 0.743 & 0.744 & 0.711 \\
3901 \& 3905 & 0.494 & 0.545 & 0.552 & 0.440 & 0.518 & 0.636 & 0.829 & 0.806 & 0.811 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Correlation coefficients between SPIs and NDVI during the peak growing season. during the peak growing season. Finally, we used a simple SIF-based model to monitor and assess the drought in the Great Plains in the summer of 2012. We found that the SIF-based model successfully tracked the spatial patterns and temporal vibration of SPI-3. We found more severe reduction of plant production in grassland than in cropland areas. SIF declined more significantly than NDVI during the peak growing season. Yet for senescence, during which time the reduction of NDVI still went on, the reduction of SIF was eased. We examined the response of SIF to drought over long time periods by analyzing GOME-2 SIF and drought indices, and then used SIF to monitor and assess drought. This work highlighted the benefits of using space-borne SIF to monitor and assess drought and better understand the relationship between large-scale drought and its influence on vegetation production. Further research should focus on the development and validation of a more advanced drought monitoring model, considering both meteorological and agricultural factors. The data used in this study were provided by the Goddard Space Flight Center of the National Aeronautics and Space Administration (NASA), the NCDC, and the AmeriFlux website. The KFS and KON AmeriFlux stations are supported through AmeriFlux core site funding from the U.S. Department of Energy under a sub contract from DE-AC02-05 CH1123. This work was supported in part by the National Natural Science Foundation of China under Grant 41501394, the Open Fund of State Key Laboratory of Remote Sensing Science under Grant Y4Y00100 KZ, the Chinese Carbon Monitoring Satellite Key Plan for Terrestrial Ecosystem under Grant Y5H0630034, the Direct Youth Foundation of the Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences (grant No. Y5 SJ2000 CX), and the China Postdoctoral Science Foundation (grant No. 2014M550871). [PERSON] designed and conducted the experiments, and wrote the whole manuscript; [PERSON] gave constructive suggestions on experiment design and the arrangement of the chapters; [PERSON], [PERSON], and [PERSON] reviewed the manuscript and gave useful advice; [PERSON] supervised the research and provided useful suggestions all through the preparation of the manuscript.