## 3 Results
### Capability for Monitoring Water Stress via Diurnal Variation Characteristics  
To investigate the ability to monitor water stress via the diurnal variation characteristics of different canopy remote sensing observations, we assessed the ability of the NMR and AMR to reflect water stress changes. Figure 15a,b shows the trends in the NMRs of the NDVI, SIFy, \(\Phi_{\rm F}^{\rm FCVI}\), and \(\Phi_{\rm F}^{\rm NIRv}\) in response to RSM changes across the different irrigation treatments throughout the observation period. Specifically, the NMR values of \(\Phi_{\rm F}^{\rm FCVI}({\rm NMR}_{\Phi_{\rm F}-\rm FCVI})\), \({\rm NMR}_{\rm NDVI}\), and \({\rm NMR}_{\rm SIFy}\) all decreased with decreasing RSM. However, the trend in the NMR of \(\Phi_{\rm F}^{\rm NIRv}({\rm NMR}_{\Phi_{\rm F}-\rm NIRv})\) diverged from the trend in RSM changes, as observed in the irr2 plot from DOYs 222 to 231 and in the irr1 plot from DOYs 222 to 225. Among these, \({\rm NMR}_{\rm NDVI}\) and \({\rm NMR}_{\Phi_{\rm F}-\rm FCVI}\) showed the most gradual decline, whereas \({\rm NMR}_{\rm SIFy}\) decreased more steeply. Following rewatering, there was a notable increase in the RSM value, which coincided with a significant increase in \({\rm NMR}_{\rm SIFy}\),\({\rm NMR}_{\rm NDVI}\), and \({\rm NMR}_{\Phi_{\rm F}-\rm FCVI}\). The diurnal variation characteristic NMR can accurately reflect the degree of vegetation water stress, whereas the ability of different canopy remote sensing observations to reflect water stress via the NMR varies. The AMR can also reflect changes in water stress, but its ability differs from that of the NMR. Figure 15c,d shows the variations in the AMR index with changes in the RSM. Owing to the occurrence of a cloudy afternoon on DOY 226, the changes in sunlight were very rapid. Hence, the AMR values for that day were unreliable. The response of the AMR to RSM changes was less sensitive than that of the NMR. In the irr2 plot, water stress was higher than that in the irr1 plot. Consequently, except for the AMR of \(\Phi_{\rm F}^{\rm NIRv}({\rm AMR}_{\Phi_{\rm F}-\rm NIRv})\), the AMRs of \(\Phi_{\rm F}^{\rm FCVI}({\rm AMR}_{\Phi_{\rm F}-\rm FCVI})\), \({\rm AMR}_{\rm NDVI}\), and \({\rm AMR}_{\rm SIFy}\) all decreased with decreasing RSM. However, for the irr1 plot with relatively mild water stress, the changes in the AMR were not significant, except for \({\rm AMR}_{\rm SIFy}\). Therefore, the diurnal variation characteristic (AMR) exhibits a lower sensitivity for early water stress detection than that of the NMR. Since the midday (12:00 to 14:00) mean canopy-air temperature difference (\(\Delta\)T\({}_{\rm moon}\)) can reflect changes in water stress, we also used this difference to evaluate the ability of the NMR and AMR of different canopy remote sensing observations to monitor water stress. Figure 16a shows the correlations and slopes between \(\Delta\)T\({}_{\rm moon}\) and \({\rm NMR}_{\rm SIFy}\), \({\rm NMR}_{\rm NDVI}\), \({\rm NMR}_{\rm P-\rm FCVI}\), and \({\rm NMR}_{\Phi_{\rm F}-\rm NIRv}\) on clear days over the observation period across the two plots. Among these, the coefficient of determination (R\({}^{2}\)) represents the degree of correlation between the changes in the index and water stress, while the slope (a) indicates the range of changes in different indices under the same water stress conditions, characterizing  
Figure 15: Variations in the NMR and AMR of the NDVI (black), SIFy (green), \(\Phi_{\rm F}^{\rm FCVI}\) (purple), and \(\Phi_{\rm F}^{\rm FCVI}\) (brown) in response to changes in the RSM on clear days throughout the observation period (DOYs 222–234). Panels (**a**,**b**) and (**c**,**d**) show the irr1 and irr2 plots, respectively. The red and blue dashed vertical lines indicate the rewatering times for the irr1 and irr2 treatments, respectively. the sensitivity of the index to water stress.