Performance evaluation of a low-cost thermal camera for citrus water status estimation

The main limitation of conventional methods generally used for monitoring the crop water stress lies in the expenditure of time and laboriousness (e.g., stem water potential, Ψstem). In this sense, infrared thermography can assist to identify the crop water status for precise irrigation purposes. However, this method requires high cost equipment and heavy systems for real-time analysis of the results. The aim of this research was to evaluate the reliability of a portable low-cost sensor (FLIR One Pro), connectable to a smartphone, for determining the water status of orange trees subjected to different treatments (i.e., full irrigation versus regulated deficit irrigation with or without soil mulching). The thermal information obtained from FLIR One Pro was compared with the data acquired with a professional thermal camera (Optris Xi 400) for two consecutive years (2021–2022). In addition, the reliability of the low-cost sensor was assessed, in respect to the loss of accuracy due to the sensor's price reduction, in identifying the crop water stress index (CWSI) and the upper and lower baselines. A good agreement was obtained between the canopy temperature (Tc) and the references leaves temperatures (dry leaf, Td; and wet leaf, Tw) provided by both sensors, resulting in coefficient of determination (R2) of 0.89, 0.82 and 0.75, respectively. The CWSI comparing the two sensors provided a R2 of 0.75. No influence of the agrometeorological conditions and overheating of the low-cost sensor on the Tc was found, however it is advisable to keep the low-cost sensor under homogeneous weather conditions during the acquisition process. No correlation was found between the CWSI and the Ψstem, due to the isohydric behavior of citrus species. Finally, this study opens new insights for spreading the use of low-cost thermal sensors for speeding up the crop water status monitoring under field conditions and supporting the adoption of precision irrigation criteria.