## 4 Discussion  
### Potential of Spaceborne SIF for Drought Monitoring over Water-Limited Ecosystems  
Satellite-based SIF observations capture the spatial and temporal variations in dryland vegetation under 2018-2019 extreme drought in Australia (Figures 4 and 5). Relative  
Figure 9: (**a-h**) Relationship between tower-based GPP and satellite-based SIF, EVI in 2018–2019 across five flux tower sites at different spatial (0.5\({}^{\circ}\), 0.05\({}^{\circ}\) grid) and temporal (monthly, 16-day) resolution. The bottom-right embedded box shows the legend of the five flux tower sites. (\(p\) value \(<\) 0.001). to EVI and GPP, the fluctuated SIF series and notable spatial speckling were probably due to high retrieval noise in sparsely vegetated regions [54], especially for lower signal levels under extreme drought. Overall, SIF and EVI exhibited resemblance to spatial and temporal responses of dryland vegetation to this extreme event, wherein the central study area (between 17-20\({}^{\circ}\)S and 129-138\({}^{\circ}\)E) showed the most remarkable reduction (Figure 5). The primary difference in spatial dynamics of two variables occurred over the northern humid region (between 12-15\({}^{\circ}\)S and 129-138\({}^{\circ}\)E) under medium drought, where SIF showed marginally negative anomalies in comparison to positive EVI anomalies (Figure 5). This is consistent with previous studies [32; 34; 36; 55], revealing that there was a considerable decoupling of photosynthesis and greenness dynamics under moderate dry conditions over mesic ecosystems (e.g., forests and woodlands). Nonetheless, there was concurrently substantial loss in both SIF and EVI over arid and semiarid biome types, and those suffered from extreme rainfall deficit as well as severe heat wave (Figure 5). In particular, a higher percentage of the area indicated by EVI was affected by extreme (\(<-2\sigma\)) losses than that by SIF throughout the entire 2018-2019 monsoon season (Figures 5 and 8). This finding was against a similar study [38], which concludes satellite SIF observations are more sensitive to water and heat stress than EVI over arid central Australia in the 2019 heatwave. However, the divergent results were primarily ascribed to the different methods to define the anomaly, and they utilized the relative anomaly (a departure from the climatology and divided by the multiyear mean) instead of the standardized anomaly used in this study. The larger variation in magnitude of SIF than that of greenness-based VIs gave rise to the sharper reduction in SIF, indicated by relative anomaly relative to EVI. In addition, insufficient baseline years (2015-2018) in their study may induce more uncertainties as a consequence of highly dynamic climate regime over inland Australia. On the contrary, we contrasted the responses of two sources of SIF data derived from GOME-2A and GOME-2B with MODIS EVI under two reference climatologies (2007-2018 and 2014-2018), respectively. We found that the percentage of the drought-affected area captured by EVI was consistently higher than that of SIF regardless of diverse temporal scales (Figure 8). The reason for the more significantly negative standardized anomalies of EVI relative to those of SIF was possibly owing to the fact that the dramatically drought-induced reduction in vegetation leads to barely increased magnitude of seasonal cycles over the vast inland area (Figure 9). Apart from this, high-retrieval noise provokes the larger variability of SIF signal over arid/semiarid ecosystems. Compared with coarse SIF data derived from GOME-2, the state-of-the-art TROPOMI SIF observation with substantially improved spatiotemporal resolution shows an enhanced correlation with tower-based GPP in the extremely dry year of 2018-2019 (Figure 9), implying that this advanced dataset has promising potential for drought monitoring over heterogeneous arid and semiarid ecosystems.