## 2 Materials and Methods  
### Description of the Experiment  
The experimental plot is located in Gucheng, Baoding city, Hebei Province, China (115 deg44'36\"E, 39 deg926\"N). The region is characterized by a temperate continental climate, with maize as one of the primary crops cultivated from mid-June to late September. The experiment involved two plots, each measuring 2 \(\times\) 4 m (Figure 1c). To prevent water seepage between adjacent plots, a 2.5 m-deep concrete barrier encircles each plot. Additionally, movable rain shelters were installed to protect the experimental area from natural precipitation, thus ensuring controlled water management throughout the study.  
In this field experiment with maize, the Zhengdan 958 variety was sown on the day of year (DOY) 170 in 2023. At the early growth stages of the maize, irrigation was precisely controlled to maintain similar soil moisture conditions across all the experimental plots, thus ensuring consistent initial growth conditions. We collected soil samples from a depth of 50 cm to calculate the relative soil moisture (RSM). RSM values were determined by employing the ratio of gravimetric soil moisture to the soil moisture holding capacity. This specific RSM calculation method was based on methodologies outlined in previous research [24]. Figure 2 shows the RSM and irrigation volume during the observation period. It is noteworthy that we implemented different irrigation schemes on two plots, with the plot subjected to Irrigation Scheme 1 named irr1 and the plot subjected to Irrigation Scheme 2 named irr2. Each plot underwent its own control process, transitioning from a well-watered state gradually to drought conditions, and then back to rehydration to reach a non-water stress control state again. This water-controlled experiment, which simulates multiple cycles of gradual progression from non-drought to drought and subsequent drought recovery, demonstrates the reproducibility of vegetation diurnal variation patterns in response to changes in water stress, thereby supporting the reliability of the study's findings (Figure 1c). The maize observation period extended from DOY 222 to DOY 234. The irr1 plot was rewetered on the evenings of DOY 225 and DOY 232, whereas irr2 received rewatering on the evening of DOY 233 to alleviate maize water stress. Based on the RSM values, the daily drought levels for each plot are classified into three categories: non-drought (Class I, RSM > 60), moderate drought (Class II, 40 < RSM \(\leq\) 60), and severe drought (Class III, RSM \(\leq\) 40) [26]. The daily drought status of each plot is detailed in Table 1.  
Figure 1: (a) AutoSIF canopy observation system and temperature sensor, (b) Monitoring-PAM probe, and (c) aerial view of the study area.  
Figure 2: Variation trends of the relative soil moisture (RSM) and irrigation amount in the irr1 and irr2 plots throughout the observation period.  
Additionally, two SI-411 infrared radiometers (Apogee Instruments Inc., Logan, UT, USA) were installed above the irr1 and irr2 plots to monitor the canopy temperature. The installed infrared temperature sensors provided a field-of-view angle of 44\({}^{\circ}\), an absolute accuracy greater than 0.2 \({}^{\circ}\)C, and monitoring data at 5 min intervals.  
Meteorological observations, including the air temperature (Ta), total radiation (Total Rad), and relative humidity (RH), were measured, and the vapor pressure deficit (VPD) was derived from the RH and Ta [27]. These meteorological observations were collected at a frequency of one sample per minute, with 10 min average values analyzed. All observational data and their attributes used in this study are listed in Table S1.