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• CWSI is first examined to detect water status of ecosystem in the humid conditions. • The CWSI dTe fail to monitor water status of cork oak plantation. • The work discussed the reason for the failure in establishment of NWSB. • The CWSI dTt could monitor water status in the plantation. • The optimum monitoring time was 13:00-14:00. The crop water stress index based on canopy–air temperature differences (CWSI dT) is an appropriate index for monitoring the water status of croplands but is rarely used to evaluate the water status of forest ecosystems. In this study, we selected a cork oak plantation in the lithoid mountain area of northern China as the research object and observed the canopy temperature (T c), actual evapotranspiration (ET a), and meteorological factors of the plantation continuously and synchronously during the period of canopy closure (May–September) in 2020 and 2021. Taking the crop water stress index based on the ratio of ET a to potential evapotranspiration (CWSI et) as a standard, the applicability of CWSI dT for detecting water status in the plantation was evaluated. The results showed that, under well-watered conditions, the canopy–air temperature differences (dT) and the canopy surface transpiration rate were both controlled by the net radiation received by the canopy surface (R nc), and the warming effect of R nc masked the cooling effect of latent heat on dT , resulting in a failure establishment of the non-water-stressed baseline in the plantation. Therefore, the empirical CWSI dT (CWSI dTe) is not suitable for evaluating the water status in this plantation. However, the theoretical CWSI dT (CWSI dTt), based on the energy balance and Penman–Monteith equation, captured daily variations in the water status in our plantation and showed a significant linear correlation with the measured relative available water content in the root zone (R2 = 0.59, P < 0.01) and CWSI et (R2 = 0.75, P < 0.01). The CWSI dTt showed an upward trend during the day, and midday (13:00−14:00) was the optimal time to measure dT for detecting the daily water status of the plantation. The CWSI dTt provides an effective, sensitive, and non-contact method of assessing daily water status changes in the cork oak plantation. [ABSTRACT FROM AUTHOR]