## 5 Conclusions  
This study demonstrated, via controlled hydration experiments of maize canopies, the effectiveness of using the diurnal variations in canopy structure and physiological remote sensing observations to monitor daily water stress levels. These observations include measurements at both the leaf and canopy scales. It was investigated whether SIFy, which integrates diurnal changes in both the canopy structure and physiology, is more effective or if \(\Phi\)F\({}_{\text{canopy}}\), which solely reflects physiological changes, is preferable. The results demonstrated that the consideration of the diurnal variation characteristics of the NMR can eliminate the influence of historical stress, thus facilitating the effective monitoring of the water stress level on the measurement day. This occurs because vegetation in the morning is less affected by water stress, providing stable information such as the LAI and chlorophyll content, which reflects the impact of historical stress. Conversely, the values obtained at noon primarily represent the changes in vegetation due to water stress on the measurement day. Remote sensing observations that integrate both canopy structure and physiological information, such as NMR\({}_{\text{SIFy}}\), are more suitable for monitoring the daily water stress status. Furthermore, we confirmed that the diurnal variation trends of \(\Phi_{\text{F}}^{\text{NIRv}}\) did not significantly change under varying conditions of water stress in maize canopies. Therefore, we recommend the use of the NMR of remote sensing observations that integrate both vegetation structure and physiological information to monitor daily water stress levels. In summary, the diurnal variations in the vegetation structure and physiology exhibit significant potential for accurately monitoring daily water stress levels and eliminating the influence of non-drought factors. This information is crucial for understanding the development of drought and implementing precise irrigation strategies.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs17040565/s1](https://www.mdpi.com/article/10.3390/rs17040565/s1)]([https://www.mdpi.com/article/10.3390/rs17040565/s1](https://www.mdpi.com/article/10.3390/rs17040565/s1)).  
Conceptualization, [PERSON] and Z.L.; methodology, J.W.; formal analysis, Z.L.; investigation, J.W. and H.J.; resources, J.W., H.J., H.Z., T.G., R.Z., D.H. and H.Z.; writing--original draft preparation, J.W.; writing--review and editing, J.W., Z.L., P.Y., S.X. and H.J.; visualization, J.W.;supervision, Z.L.; project administration, Z.L.; funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Foundation of China (Grant No. 42471379, 42071402) and National Key Research and Development Program of China (Grant No. 2023 YFF1303603).  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
We thank the anonymous reviewers for their constructive comments. We are also grateful to the Natural Science Foundation of China and National Key Research and Development Program of China for their support.  
The authors declare no conflict of interest.