The influences of canopy temperature measuring on the derived crop water stress index

Crop water stress index (CWSI) is widely used for efficient irrigation management. Precise canopy temperature (T_c) measurement is necessary to derive a reliable CWSI. The objective of this research was to investigate the influences of atmospheric conditions, settled height, view angle of infrared thermography, and investigating time of temperature measuring on the performance of the CWSI. Three irrigation treatments were used to create different soil water conditions during the 2020–2021 and 2021–2022 winter wheat-growing seasons. The CWSI was calculated using the CWSI-E (an empirical approach) and CWSI-T (a theoretical approach) based on the T_c. Weather conditions were recorded continuously throughout the experimental period. The results showed that atmospheric conditions influenced the estimation of the CWSI; when the vapor pressure deficit (VPD) was > 2000 Pa, the estimated CWSI was related to soil water conditions. The height of the installed infrared thermograph influenced the T_c values, and the differences among the T_c values measured at height of 3, 5, and 10 m was smaller in the afternoon than in the morning. However, the lens of the thermometer facing south recorded a higher T_c than those facing east or north, especially at a low height, indicating that the direction of the thermometer had a significant influence on T_c. There was a large variation in CWSI derived at different times of the day, and the midday measurements (12:00–15:00) were the most reliable for estimating CWSI. Negative linear relationships were found between the transpiration rate and CWSI-E (R² of 0.3646–0.5725) and CWSI-T (R² of 0.5407–0.7213). The relations between fraction of available soil water (FASW) with CWSI-T was higher than that with CWSI-E, indicating CWSI-T was more accurate for predicting crop water status. In addition, The R² between CWSI-T and FASW at 14:00 was higher than that at other times, indicating that 14:00 was the optimal time for using the CWSI for crop water status monitoring. Relative higher yield of winter wheat was obtained with average seasonal values of CWSI-E and CWSI-T around 0.23 and 0.25–0.26, respectively. The CWSI-E values were more easily influenced by meteorological factors and the timing of the measurements, and using the theoretical approach to derive the CWSI was recommended for precise irrigation water management.