Assessment of UAV thermal imagery to monitor water stress in olive trees

The optimization of water use is essential in a climate change scenario, in which this natural resource will become increasingly scarce. To face this challenge, deficit irrigation has been used as a strategy in different crops. However, the adoption of this strategy requires a deep knowledge of the culture’s response in the presence of water scarcity to ensure the sustainability of production and keep the plant free from danger. In recent decades, remote sensing techniques have evolved to provide fast and effective ways to monitor water status indicators using thermal infrared sensors, capable of covering considerable areas. This cannot be achieved by current ground-based sampling techniques. The crop water stress index (CWSI) is the most common index used for monitoring and quantifying water stress as well as for irrigation scheduling. However, the correlation between CWSI and water status indicators measured in the field, needs to be modulated by each crop. The main goal of this study is to assess the applicability of the CWSI for the estimation of water status indicators, such as, relative water content (RWC), midday leaf water potential (Ψ_MD) and stomatal conductance (g_s) in olive trees (Olea europaea L.) of ‘Cobrançosa’. Moreover, a critical analysis of the stomatal conductance index (I_g) is also performed. Different irrigation strategies were imposed in the study area, an olive orchard located at Vilariça Valley, Alfândega da Fé, northeastern Portugal. CWSI showed a good correlation with all the water status indicators used in this study (r²>0.79), achieving the best performance with stomatal conductance (r²=0.87). Thus, CWSI appears as a powerful and valuable tool for monitoring water stress in olive trees.