Estimating Maize Water Stress Using High-Resolution Thermal Imagery.

In this study, we developed a new crop water stress indicator, standard deviation of canopy temperature within thermal imagery (CTSD), to monitor crop water status. Thermal imagery was taken from maize (Zea mays L.) plants under various levels of deficit irrigation in different crop growth stages. The canopy temperature distribution was obtained from thermal imagery by the Expectation-Maximization algorithm under a range of canopy coverage and water stress conditions. Several other crop water stress indicators were measured or calculated in 2012, 2013, 2015 and 2016 to compare with the CTSD, including soil water deficit (SWD), leaf water potential (ψ), stomatal conductance, and crop water stress index. Based on canopy energy balance model, we proved that the temperature differences between sunlit and shaded parts of the canopy would increase with the increasing of canopy resistance in the sunlit part of crop canopy. The CTSD well described crop water stress dynamic throughout the growing season, and responded to irrigation events. The CTSD showed statistically significant relationships with all other water stress measurements. The result suggests that the canopy temperature standard deviation could be a reliable indicator of crop water status. The CTSD value smaller than 2.5 indicates a non-water stress condition. Because CTSD only relies on the canopy temperature itself, it has strong application potential. Moreover, it may also be applied to high resolution thermal imagery from other remote sensing platforms, such as unmanned aerial vehicles.