## 4 Conclusions and Discussion  
In this paper, based on the satellite EVI and SIF products along with other climate and vegetation datasets, we examined the impacts of drought on the vegetation photosynthesis for crops and forests in Southeast Asia during the 2015/16 El Nino event. We found that satellite SIF observations were more sensitive than traditional VIs to monitor water stress for both evergreen broadleaf forest and croplands in Southeast Asia. Therefore, the space-borne SIF can be used as an effective indicator to predict the response of vegetation to water and heat stresses in the tropical region. Previous studies have reported that the higher solar radiation motivates leaf flush and leaf expansion in forests [35; 55; 56]. Leaf area index could be first enhanced because of the increased solar radiation, and then EVI. As a result, two regions in Southeast Asia showed green up when the drought happened according to the increases of EVI anomalies, which was related to the higher radiation (Figures 3-5). Similarly, SIF cannot characterize drought well compared to SIF\({}_{\text{yield}}\) in the late 2015 in both areas because of high radiation, but it still showed higher sensitivity to drought than EVI. SIF showed more negative anomalies than EVI (Figure 4) because SIF was directly linked to the vegetation functioning, indicating that SIF can potentially be used as proxy for monitoring vegetation droughts with different onset mechanisms. Moreover, the impacts of drought on SIF became more pronounced when the effect of radiation was removed (Figures 5 and 6). This phenomenon was observed for both vegetation types. Previous studies have demonstrated that the water stress altered the LAI during the drought [30]. As we know,  
Figure 9: Monthly average of OCO-2 XCO\({}_{2}\), SIF for the Southeast Asia from 2015 to 2017. LAI is an important canopy structure parameter and has a direct impact on the fraction of absorbed photosynthetic active radiation (fPAR). [PERSON] et al. [PERSON] et al. (2018) used a generalized radiative transfer model within the canopy to investigate the relationship between fPAR and LAI, and the results demonstrated that fPAR increased with higher LAI. Therefore, the altered fPAR and APAR during the drought may have strong impacts on radiative transfer process of SIF and the probability that SIF photons escaping from the canopy. And our results also confirmed that the drought had more pronounced impacts on SIF\({}_{\text{yield}}\) than SIF and EVI because of elimination of the effects of the solar radiation variations in SIF\({}_{\text{yield}}\). The effects of vegetation structural variables were eliminated, then important information on drought events could be provided by SIF\({}_{\text{yield}}\). Changes on vegetation greenness and chlorophyll content may not rapidly reflect the information about vegetation status under drought, but variations on the production of fluorescence have rapid responses to water stress [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018). Therefore, SIF\({}_{\text{yield}}\) is the suitable factor to detect the changes of vegetation function during the drought period in the study areas. The strong 2015/2016 El Nino provided an opportunity to investigate the response of tropical terrestrial ecosystems to extreme climatic event [PERSON] et al. (2018). Previous studies focused on the drought impacts of the 2015/2016 El Nino events in the tropics [PERSON] et al. (2018); [PERSON] et al. (2018) and figured out the mechanisms affected canopy photosynthesis. More studies demonstrated that the GPP decreased due to the extreme drought in the tropics [PERSON] et al. (2018); [PERSON] et al. (2018). The enhanced PAR did not increase GPP in Amazon during the 2015/2016 drought [PERSON] et al. (2018). Field experiments at the Amazon KM83 site convinced that strong radiation resulted in increased leaf temperature, which reduced canopy photosynthesis [PERSON] et al. (2018). [PERSON] et al.