## 3 Results
### Diurnal Variations in Canopy Spectral Observations
#### 3.3.2 Diurnal Variations in Remote Sensing Observations Related to Vegetation Physiology  
The diurnal patterns of the SIF and its derived parameters can reflect the physiological response of vegetation to water stress. As shown in Figure 8a, the SIF values throughout  
Figure 7: The diurnal variations of photosynthetically active radiation (PAR) (**a**), normalized difference vegetation index (NDVI) (**b**), near-infrared reflectance of terrestrial vegetation (NIRv) (**c**), and fluorescence correction vegetation index (FCVI) (**d**). Scatter plots were used to visualize these variations. The solid lines of different colors indicate the respective fitted curves of each index. The red and blue dashed vertical lines indicate the rewatering times in the irr1 and irr2 plots, respectively. the observation period exhibited an arch-shaped daily variation. At the early stages of water stress or following the alleviation of water stress, the trends in the SIF and PAR were consistent (irr1: DOYs 226, 237, and 228; irr2: DOYs 222 and 225). On DOY 231, the midday peak of the SIF in the irr1 plot exhibited a significantly earlier occurrence due to water stress. With increasing water stress, the midday peak of the SIF progressively decreased, and the arch shape became flatter, as observed in the irr2 plot from DOYs 225 to 231. Therefore, changes in the diurnal variation trend of the SIF can effectively reflect the degree of water stress. The diurnal variation in SIFy also exhibited high sensitivity to water stress, as shown in Figure 8b. Under well-watered conditions, SIFy exhibited minimal variation throughout the day (irr1: DOYs 227, 228, and 234). On the afternoon of DOY 226, SIFy anomalously declined due to unstable sunlight conditions (Figure 7a). Under water stress, the diurnal variation pattern of SIFy showed relatively high values in the morning, a decrease to its lowest point at noon, and recovery in the afternoon. With increasing water stress, from DOYs 222 to 231 in the irr2 plot, high morning values with slight declines were observed, with the midday nadir values decreasing progressively and exhibiting high variability. Notably, after rewatering during the night of DOY 233 in the irr2 plot and during the nights of DOYs 225 and 232 in the irr1 plot, a significant increase in the midday nadir values was observed, indicating that the diurnal variation in SIFy quickly responds to the relief of water stress. The diurnal pattern of \(\Phi_{\rm F}^{\rm FCVI}\) also varied under the different water stress conditions, but the variation was lower than that in SIF and SIFy (Figure 8d). Comparing the irr2 and irr1 plots under varying levels of water stress, \(\Phi_{\rm F}^{\rm FCVI}\) exhibited a greater range in the irr2 plot. With increasing water stress, the diurnal pattern of \(\Phi_{\rm F}^{\rm FCVI}\) shifted from a pattern encompassing an initial increase in the morning, a peak at noon, and a subsequent decline to a pattern where it started to decrease from the morning. After rewatering and alleviation of water stress, there was a notable increase in the overall value of \(\Phi_{\rm F}^{\rm FCVI}\). Therefore, the response of \(\Phi_{\rm F}^{\rm FCVI}\) to water stress was reflected mainly in the changes in its diurnal pattern and the overall decrease in its magnitude. Figure 8: The diurnal variations of solar-induced chlorophyll fluorescence (SIF) (**a**), apparent SIF yield (SIFy) (**b**), canopy fluorescence emission efficiency (\(\Phi_{\rm F}^{\rm NIRV}\)and\(\Phi_{\rm F}^{\rm FCVI}\)) (**c**,**d**). Scatter plots were used to visualize these variations. The solid lines of different colors indicate fitted curves for each parameter. The red and blue dashed vertical lines indicate the rewatering times in the irr1 and irr2 plots, respectively. However, \(\Phi_{\mathrm{F}}^{\mathrm{NIRv}}\) showed no significant differences in diurnal variations across the different levels of water stress (Figure 8c).