## 3 Results
### Differences of F760 tot, F760 toc, and VIs in Responding to Different Drought Levels
As for the differences, these mainly involved that the response time of F760 tot, F760 toc, and NIRv to droughts were obviously ahead of the NDVI in the period of the first rectangle (18-29 April), thus indicating the rapid response of F760 tot, F760 toc, and NIRv to the onset of a drought, and among them, the response patterns of F760 toc and F760 tot to a drought were quite similar (i.e., responding simultaneously to the second irrigation (22 April) on a daily scale). The newly proposed NIRv index appeared to respond more rapidly to droughts than the NDVI, but the magnitudes of its values of P1 and P2 were opposed to the different drought levels between them, and the order of NIRv values matched with the drought levels until the third irrigation. That aside, a rapid response difference between F760 tot, F760 toc, and the NDVI for the irrigation-induced variation of SM was also observed. For instance, a sudden rise happened with F760 tot and F760 toc of the four plots after irrigation, especially in the second irrigation (22 April to 20 May), while the values of the NDVI showed a smooth trend, indicating an accumulative effect of the drought. Conclusively, it could be inferred that both F760 tot and F760 toc had an edge over the VIs in the early detection of droughts. Additionally, the seasonal patterns of AF76 toc and AF76 tot (SIF/PAR, normalized by PAR) under different intensities of droughts were also analyzed. Overall, SIF/PAR showed similar trends of change with those of SIF (Figure 5); that is, its values for the four plots were generally directly proportional to the drought levels. Specifically, as shown in Figure 5a,b, the fluctuation of SIF/PAR was reduced after eliminating the effect of PAR on SIF. In the first rectangle of two irrigations, the AF760 values of the four plots were displayed in the order of P2 > P1 > P3 > P4 for AF760 toc and P1 > P2 > P3 > P4 for AF760 tot. In the second rectangle of the four irrigations, the AF760 values of the four plots were shown in the order of P1 > P2 > P3 > P4 for AF760 toc and P1 > P4 > P2 > P3 for AF760 tot. Figure 5: Seasonal patterns of **(a)** AF760 toc and **(b)** AF760 tot. The color and style of the dot lines and rectangles are the same as Figure 4, and the black dash-dotted lines in panels (a–c) indicate the irrigation for four plots. In order to further investigate the impact of F760 tot on droughts, the drought-induced decreases, especially those caused by SM, in the NDVI, NIR\({}_{\rm v}\), F760 toc, and F760 tot between P1 and the other plots are explored in Figure 6. The theoretical basis for this analysis was that the field experiment took the SM as the control condition, and other properties such as the illumination, temperature, and wheat of the four plots were basically similar. Therefore, the differences for P1 and the other plots directly represented the impact of SM droughts on different variables. As shown in Figure 6, the drought-induced decreases of the four variables gradually increased along with the aggravation of the drought, ranging from 40% to 90%. In the same intensities of droughts, the drought-induced decreases of the four variables also gradually increased as the drought progressed, except for F760 tot in an extreme drought. As for the NDVI, it exhibited a later response to the onset of a drought than other variables and decreased more dramatically for the long-term drought than SIF, particularly in extreme droughts. As for NIR\({}_{\rm v}\), it seemed to have a compromise in performance between the SIF and NDVI in early drought monitoring, and among these four variables, the drought-induced decreases of NIR\({}_{\rm v}\) and the NDVI were gradually higher than those of SIF, whereas in a moderate drought, the NIR\({}_{\rm v}\) values of P2 were conversely higher than those of P1. Given the SIF variables, the F760 tot values decreased more than other variables when in response to the onset of a moderate drought, and the decreases of F760 tot were slightly less than those of F760 toc.