## 3 Results  
### Differences of F760 tot, F760 toc, and VIs in Responding to Different Drought Levels  
In Section 2.1, a dataset containing different types of measurements was obtained for subsequent analysis. As illustrated in Figure 3, there was a steady increase in these three parameters, including the VPD, Ta, and SR, during the growing season, among which Ta appeared to have the greatest increment (Figure 3b). Specifically, these three parameters showed the daily variability, and the peaks of Ta were well correlated with those of the VPD (13-17 April, 27 April to 1 May, and 5-9 May in 2017, etc.). Most importantly, the four white dash-dotted lines marked in panels (a-c) (8 April, 19 April, 25 April, and 3 May) indicate several valleys of SR during the growing season, which had implications for deciphering the minimum values of F760 tot (Figure 4b,c). Figure 3: Overview of the meteorological conditions during the entire measurement campaign (from 28 March 2017 to 19 May 2017): (**a**) vapor pressure deficit (VPD, kPa), (**b**) air temperature (Ta, \({}^{\circ}\)C), and (**c**) solar radiation (SR, W/m\({}^{2}\)). White dash-dotted lines in panels (**a–c**) show several valleys of SR during the growing season. In order to discuss the responses of F760 tot to different drought levels over four plots, here, the seasonal patterns of F760 tot in comparison with F760 toc, the NDVI, and NIR\({}_{\
u}\) were analyzed to illustrate the detailed relationships of these parameters with drought development. As shown in Figure 4, the drought levels of the four plots (P1 > P2 > P3 > P4 from wet to drought) was successfully established, thus supporting exploring the utility of F760 tot for drought monitoring according to [PERSON] et al. [14]. It was clearly observed that the F760 tot, F760 toc, and VIs showed different seasonal patterns; that is, the mean values of F760 tot as well as F760 toc were generally consistent with a trend of going up slightly first (27 March to 24 April) and then going down slightly (24 April to 20 May), and some valleys for F760 tot and F760 toc were perfectly matched with the low values of SR as mentioned above (Figure 4a-c), indicating the potential links between them. The VI values were characterized as a smoothly declining trend during the growing season  
Figure 4: Seasonal patterns of **(a)** PAR, **(b)** F760 tot, **(c)** F760 toc, **(d)** NIR\({}_{\
u}\), **(e)** NDVI, and **(f)** SM and irrigation. Each dot line represents the daily mean value of the measurements, with blue dots for plot 1 (P1) and orange to dark black for plots 2–4 (P2–P4). Gray transparent rectangles in panels **(a–c)** indicate the different response period of SIF and VIs to different drought levels. (Figure 4d,e). The different seasonal patterns suggested that the SIF emissions of the wheat were susceptible to the influences of various factors compared with the VIs. Given the different drought levels, there were similarities and differences between the performance of F760 tot as well as F760 toc and the VIs in the four plots. As for the similarities, the values of all variables of the four plots exhibited a difference in gradient consistent with the drought level (P1 > P2 > P3 > P4) in the growing season after irrigation, such as in the period of the second rectangle (11-20 May). These results indicate that all variables had the potential to respond to the irrigation-induced variations in SM during sustained and prolonged droughts. However, on the contrary, the F760 tot values of P4 were higher than its values for P1 in the period of the second rectangle (Figure 4b), which may have been related to premature yellowing of the wheat leaves during long-term droughts.