Optimal Time-of-Day for Thermal Remote Sensing of Water Stress in Olive Orchards.

Water management of olive orchards, where trees are submitted to stress conditions at specific phenological stages, is expected to enable optimization of high yields with high quality oil.  In order to achieve this, crop water status must be assessed accurately and reliably at the orchard scale.  Indirect large-scale measurements of water stress in plants that provide high spatiotemporal resolution are often based on remote sensing of leaf temperature.  One of the advantages of the remote sensing method is that a snapshot of thermal and true color images can be taken instantaneously and provide simultaneous information regarding large parts of the orchard.  The time at which this snapshot is acquired has an effect on the stress detection.  The objective of this work was to determine the time of image acquisition for optimal detection of water stress in olive trees.

In a field experiment in Israel where trees are irrigated with increasing relative quantities of water (30, 50, 75, 100 and 125% of potential evapotranspiration), remote thermal and true color images of individual trees were acquired, and crop water stress index (CWSI) was computed.  In addition, soil water content and potential, mid-day stem water potential, and stomatal resistance were measured. 

In the late morning, trees of all treatments had a lower CWSI compared to trees of the same treatment sampled towards noon and early afternoon, with a larger increase in CWSI for the stressed treatments.  The time at which differences between treatments are larger, and therefore better detected, is the time of maximum stress.  This means that, within the timeframe examined in this study, the best time for image acquisition is early afternoon.