## 4 Discussions
### Pros and Cons of F760 tot for Drought Monitoring
[46] proposed a framework for parametrizing the water stress function using SIF in the AgroC crop growth model and improved the estimates of actual evapotranspiration during the stressed periods. In terms of the two results, it was noted that F760 tot could respond rapidly to droughs, and the NDVI was more sensitive to long-term water deficits. The main reason for this is that SIF is strongly correlated with photosynthesis and serves as an indicator of photosynthetic activity [47], thus prompting SIF to respond near-instantaneously to rapid adjustments in the photosynthetic machinery [48] and highlighting its potential to improve the present drought detection approach [49]. From the physiological perspective, there are three possible fates for solar radiation absorbed by plant chlorophyll molecules, including photosynthesis (PQ), heat (NPQ), and emitted fluorescence, and the relationship between them is generally competitive but actually neither unique nor simple [50]. For example, the relationships between steady state fluorescence and photochemistry under low and high light (or light-statured conditions with increasing irradiance and moisture) stress were a negative and positive correlation, respectively [51]. Under prolonged drought conditions, a decrease in SM (i.e., a decrease in soil water potential) would induce a decrease in plant water potential in order to absorb as much water as possible from the soil. Moreover, plants need to decrease transpiration to limit their water loss by regulating the opening and closing of the stomata on the leaf surface. However, the closing of the stomata prevents plants from taking up carbon dioxide (CO\({}_{2}\)), thus limiting the dark reaction of photosynthesis and reducing the photosynthetic rate. It was expected that drought stress significantly decreased the leaf photosynthesis, and here, the SIF emissions also reduced along with the drought levels (Figure 4). Therefore, the relationship between photosynthesis and fluorescence has a positive correlation. Plants can decrease the rate of photosynthesis rapidly in response to a drought, which similarly reduces SIF values quickly based on the positive and strong correlation between them. In contrast, the NDVI was mainly influenced by many factors, such as the chlorophyll content, LAI, and other indirect drivers such as the total plant cover, biomass, plant, and SM. These factors tend to decline gradually under prolonged drought conditions and lead to the late response of NDVI to SM changes reflecting the accumulative effect [52]. There is one important variable of SIF normalized by PAR (AF760) that provides a better understanding of their contributions to F760. It can be found that in extreme droughs (P4), the correlation between AF760 and SM was enhanced at different time scales and time windows compared with F760 (Figures 9-11). This finding proves the necessity of normalizing SIF by PAR in drought monitoring. Moreover, the correlation between AF760 toc and the growth parameters (Chl and LAI) was enhanced (R\({}^{2}\) from 0.4 to 0.85 and R\({}^{2}\) from 0.45 to 0.65, respectively), as well as the degree of enhancement improving along with the severity of the drought, which may explain the better correlation between AF760 toc and SM than that of F760. This coincides with the findings by [PERSON] et al. [53] that SIF normalized by PAR can improve the net impacts of droughs on SIF.