## 3 Results
### Relationships of Root Zone Soil Moisture with F760 tot  
As mentioned in the previous section, the PAR is the common driving force for the dynamics of SIF, whereas in Figure 7, such a driving effect declined for four plots along with the drought levels, thus indicating the involvement of SM in SIF variations. Pearson analysis was then applied to further investigate the relationships between F760 tot with SM in different time scales (Figure 9). On the daily and hourly scales, it was obvious that the impacts of SM on F760 tot and the VIs were becoming close, along with an increase in drought stress. For P1 with no drought, SM seemed to have little influence on F760 tot and the VIs. As the drought intensified from P2 to P4, the response of F760 tot and the VIs tended to be different. The correlations between the NDVI and NIR\({}_{v}\) (r values: 0.68 and 0.75) with SM were positively much greater than those with SIF. As for SIF, only AF760 toc exhibited the most positive relationship with SM (r values: 0.54 for P4), and the reason for this may lie in that there were strong correlation relationships between AF760 toc and the VIs. Figure 8: Partial correlation coefficients (r) of (**a**,**b**) with F760 toc, F760 tot, AF760 toc, AF760 tot, NDVI, and NIR\({}_{v}\) based on daily measurements. Significance was performed according to two-tailed test (ns: \(p>\) 0.05. * \(p<\) 0.05. ** \(p<\) 0.01. *** \(p<\) 0.001). The previous analysis was conducted using the whole data from the growing season. Furthermore, taking P4 as example, the relationships between F760 tot and the VIs with SM in the 20-cm layer under different time windows from 3 days to 28 days (Figure 10) were explored, aiming to reveal the distribution of correlation coefficients under a drought. There were similarities and differences between the performances of F760 tot and the VIs with SM of P4 (extreme drought). It was confirmed that the correlation coefficients of different parameters showed the larger distribution of numerical data in the short time window and became concentrated along with the increase of time steps. Meanwhile, different variables containing SIF, SIF/PAR, and the VIs had their unique patterns. For example, the relationship of F760 tot and F760 toc with SM was weak, and AF760 and the VIs showed an increasing and positive trend correlation coefficient with SM. The correlations of SM in the 20-cm layer (SM) with lagged SIF (F760 toc and F760 tot) as well with lagged VIs (NDVI and NIR\({}_{\
u}\)) were also explored in Figure 11. In order to figure out their relationships under different time windows (from 1 week to 4 weeks), the r values using data from P4 representing an extreme drought were calculated. First, the correlations of SIF-SM and VI-SM presented a steady increasing trend and had little amplitude fluctuation with the increase in the time windows, and the correlations of the latter exceeded that of the former using the data over the growing season, suggesting that the VIs were more applicable for the drought lasting for a long time scale. In the time window of 1 week (Figure 11a), the r values of SIF-SM had a similar pattern to the trigonometric curve, and the r values of VI-SM increased in the range of 8 days and then kept consistent (r values > 0.75). Specifically, the r values of SIF-SM were higher than that of VI-SM when the time lag was -5 days (r values > 0.7), and the latter exceeded that of the former when the time lag was -9 days. This further indicated that SIF outperformed the VIs in early drought monitoring. In other panels, especially in Figure 11c,d, it is obvious that the VIs had a strong relationship with SM in different time lags, while F760 tot turned out to decouple with SM in time lags of about 3 or 4 weeks. Figure 9: Heatmap of the correlation matrix generated by the Pearson correlation coefficient shows the relationships of SM with SIF and VI at seasonal scale (upper panel) and diurnal scale (lower panel) in (**a**,**e**) plot 1, (**b**,**f**) plot 2, (**c**,**g**) plot 3, and (**d**,**h**) plot 4.