Applicability of the crop water stress index based on canopy–air temperature differences for monitoring water status in a cork oak plantation, northern China

The crop water stress index based on canopy–air temperature differences (CWSIdT) 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 (Tc), actual evapotranspiration (ETa), 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 ETa to potential evapotranspiration (CWSIet) as a standard, the applicability of CWSIdT 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 (Rnc), and the warming effect of Rnc 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 CWSIdT (CWSIdTe) is not suitable for evaluating the water status in this plantation. However, the theoretical CWSIdT (CWSIdTt), 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 CWSIet (R2 = 0.75, P < 0.01). The CWSIdTt 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 CWSIdTt provides an effective, sensitive, and non-contact method of assessing daily water status changes in the cork oak plantation.