## 1 Introduction
For example, a model for retrieving the spectral shape of the leaf fluorescence spectrum was proposed by [PERSON] et al. [19]. [PERSON] et al. [20] utilized the Random Forest (RF) model to estimate SIF at the photosystem level in both the red and far-red bands and improved the correlations between SIF and absorbed photosynthetically active radiation (APAR). A simple approach to estimate the \(\text{f}_{\text{esc}}\) of SIF at the far-red band was introduced based on the near-infrared reflectance of vegetation (NIR\({}_{\text{v}}\)) and the fraction of absorbed photosynthetically active radiation (fPAR) [21]. Generally, the O\({}_{2}\)-A band (-760 nm) is most commonly used for retrieving SIF results, or in other words, F760 (SIF at 760 nm). However, the fact is that only a fraction of the emitted SIF photons is observed from the top of the canopy, which is therefore called F760 toc. Using the above method, F760 toc can be used to calculate the total emitted SIF (SIFtot) at 760 nm (i.e., F760 tot), thus reflecting the total emitted SIF from the entire canopy. In this regard, some studies [22] suggested that SIFtot exhibited better performance in predicting GPP compared with SIFtoc. In contrast to these findings, [PERSON] et al. [23] actually found a decreased performance of F760 tot compared with F760 toc for GPP estimation. With respect to stress detection, SM is used to indicate drought-induced water stress, and the relationship between SIF and SM is complex. Generally, the physiological response of plants to a drought includes the stomatal and non-stomatal limitations, thus leading to the variation of photosynthetic electron transport, non-photochemical quenching (NPQ), and SIF. As the SM continuously declines until a certain threshold, the stomatal and non-stomatal limitations lead to a decrease both in photosynthesis and fluorescence. Based on many canopy-scale studies [5; 13; 14], a decrease in F760 can be observed under water stress, and F760 has a higher correlation relationship with SM than VI. Specifically, the behavior of the canopy-scale SIF under a drought is the ensemble of the changes in the physiological reaction and canopy structure [23], and among them, [PERSON] et al. [5] suggested that structural factors dominated the spatial response of SIF to water stress over the physiological reaction, while F760 tot represents the total emitted SIF at the leaf level and should be less affected by canopy structures. Therefore, it is unreasonable to speculate on the performance of F760 tot for drought monitoring based on the performance of F760 toc. That aside, only a few studies, such as [PERSON] et al. [24] demonstrated that the value of SIFtot apparently declined compared with the multi-year average under a drought, and there appeared to be similar monitoring effects for both SIFtoc and SIFtot regarding droughts. This study used satellite SIF data to obtain these findings without ground experiments. Moreover, the response time and magnitude of SIFtot (F760 tot) for droughts and its relationships with the environmental parameters and SM (i.e., the potential of F760 tot in drought detection) remain unclear. If this is figured out, the results might provide a deep insight into the utility of F760 tot in drought monitoring, thus contributing to mitigating the risk of agricultural droughts. To explore the monitoring effects of F760 tot on droughts, here, the continuous ground measurements to four plots under different levels of drought for a wheat crop were carried out.