Tree-based sap flow monitoring to validate the crop water stress index in hazelnut

Large-scale hazelnut orchards need a fast and cost-effective method to estimate plant water status to properly manage irrigation. Imagery captured by unmanned aerial vehicles (UAV) is a promising technique because canopy temperature can be related to plant transpiration. This work aims to identify a correlation between the crop water stress index (CWSI) derived from UAV thermal images and the transpiration of hazelnut in the frame of the Pantheon project. The study was set in a private hazelnut orchard located in Viterbo Province (central Italy). The orchard of Tonda Romana was planted in 2010 in a 5×5 m planting design. Plants were grown as a free vase with 9 branches per plant and a mean branch diameter of 6.9 cm. The trial included a rain fed (control) and a sub-irrigated treatment. We installed thermal dissipation probes on 4 branches per treatment. A datalogger collected the sap flow data every 15 min during 2020 together with meteorological parameters. Sap flow, tree biometrics and microclimatic parameters served to estimate stem conductance. The UAV equipped with a thermal camera flew at an altitude of about 25 m which resulted in a ground resolution of around 2.7 cm pixel‑1. The CWSI was extrapolated for the sap flow-monitored plants for 5 sample days and related to the plant conductance. Results showed that Tonda Romana has a good water saving strategy through tight stomatal closure when it is well irrigated or sufficiently rain fed. The CWSI has a promising correlation with plant conductance. In 2020 there was no lack of rain, thus sap flow sensors showed no significant differences in the irrigation trials; this agrees with CWSI results. Still UAV-based water status assessment might be more helpful in areas with lower precipitation, while sap flow sensors are more likely to catch small differences in water status.