Monitoring olive tree water status by unmanned aerial vehicles (UAVs) and trunk dendrometers

In the last decade unmanned aerial vehicles (UAVs) and plant based sensors have been proposed for the monitoring of plant water status. Thermal infrared images from UAV have been widely used for monitoring plant water status in orchards and vineyards. Among plant based sensors, linear variable differential transformer (LVDT) dendrometers allow to measure changes in diameter induced by diurnal fluctuations of transpiration in different organs. We tested the use of UAV thermal imaging and dendrometers for the estimation of olive tree water status in a commercial olive orchard (Olea europaea cvs. Frantoio e Leccino) where three irrigation regimes were established: Full irrigation (FI, 100% of ET c ), sustained deficit irrigation (DI, 40% of full irrigation) and rainfed (RF, only rainfall). The stem water potential ($\Psi_{stem}$) was measured throughout the growing season. Thermal images were acquired from an altitude of 50 m above ground level. Three trees for each cultivar were equipped with trunk dendrometers in order to measure continuously the daily trunk diameter fluctuations. The signal intensity (SI) values of $\Psi_{stem}$ and crop water stree index (CWSI) were consistent and reflected the positive and almost linear relationship existing between SWP and CWSI in olive. On the other hand, SI values of maximum daily shrinkage (MDS) were almost always lower than one. The relationship between $\Psi_{stem}$ and MDS showed a positive relationship as long as $\Psi_{stem}$ values were higher than -2 MPa, whereas a decrease in the amplitude of trunk diurnal fluctuations was observed at higher level of water deficit.