## 3 Results
### Diurnal Variations in Canopy Spectral Observations
#### 3.3.2 Diurnal Variations in Remote Sensing Observations Related to Vegetation Physiology
Apart from DOYs 226 and 231, when sunlight instability was noted, the diurnal trend of \(\Phi_{\mathrm{F}}^{\mathrm{NIRv}}\) on other days exhibited an initial increase in the morning, a peak at noon, and a subsequent decline, thereby maintaining a stable overall trend without significant changes across the different water stress conditions. This indicates that the diurnal variation in \(\Phi_{\mathrm{F}}^{\mathrm{NIRv}}\) does not effectively reflect water stress changes. Differences in the Vegetation Structure and Physiology Between the Morning and Noon Under Different Levels of Water Stress  
To analyze the differences in the diurnal variation characteristics of the vegetation structure and physiology under different water stress levels, we used the LRR to categorize vegetation water stress into two levels. The first condition, termed Mild Drought, was defined by an LRR value greater than 0.7 throughout the day, including the midday peak of environmental stress. This was observed on specific DOYs, such as 226, 227, and 231 in the irr1 plot and DOY 234 in the irr2 plot, indicating minimal or no leaf rolling and suggesting either mild or no water stress. Severe Drought was defined as a daily LRR value below 0.7 during the peak midday stress period. For example, this was observed on DOY 226 in the irr1 plot and DOYs 225, 226, and 227 in the irr2 plot, indicating significant leaf rolling and substantial changes in the leaf structure (Figure 5). 4.1 Differences in the Vegetation Structure Between the Morning and Noon Under Different Levels of Water Stress  
At the leaf and canopy scales, the LRR and the response of the NDVI to water stress, respectively, significantly differed between the morning and midday periods (Figure 9). Specifically, during the Mild Drought phase, the NDVI and LRR values exhibited minimal differences between the morning (8:00-9:00) and noon (12:00-13:00) periods, with only a slight decline at noon due to the increased environmental stress. In contrast, during Severe Drought phases, although the morning NDVI and LRR values remained relatively high, there were significant decreases by noon. Among them, the LRR showed a greater decrease at midday under severe drought conditions than did the NDVI. This likely occurred because the NDVI is a composite measure of vegetation greenness and biomass at the canopy scale, whereas the LRR directly reflects changes in the leaf structure. As a result, the LRR was more sensitive to variations under water stress. At both the leaf and canopy scales, it was noted that in the morning, the impact of water stress on the crop structure was not significant. However, at noon, the effect of water stress on the vegetation structure was notable. Figure 9: Diurnal variations in the average LRR and NDVI values at the different leaf rolling stages in the morning (8:00–9:00) and at noon (12:00–13:00). 4.2 Differences in Vegetation Physiology and Vegetation Structure Between the Morning and Noon Under Different Levels of Water Stress  
At the leaf scale, energy allocation of vegetation varies with light and water stress. Significant differences in the diurnal variation in the energy distribution within the photosystem occurred under varying levels of water stress. Figure 10 shows the diurnal variations in the energy distribution in the photosynthetic system of sunlit leaves in the irr1 plot. In the morning, when the PAR level was less than 600 \(\upmu\)mol \(\cdot\) m\({}^{-2}\cdot\) s\({}^{-1}\), the \(\Phi_{\rm N}\) value remained relatively low, typically under 0.2 between 8:00 and 9:00 (Figure 10). As the day progressed, \(\Phi_{\rm N}\) gradually increased and peaked at noon. With decreasing RSM, the midday peak of \(\Phi_{\rm N}\) notably increased (Figure 10). In contrast, \(\Phi_{\rm P}\) exhibited the opposite trend as that of \(\Phi_{\rm N}\). Notably, it was higher in the morning and decreased over time, reaching its lowest point at noon. This minimum value of \(\Phi_{\rm P}\) decreased with decreasing RSM.