Assessment of the Spatial Varability of CWSI Within Almond Tree Crowns and its Effects on the Relationship with Stomatal Conductance

Abstract

This work focuses on understanding the effects caused by the within-tree structural heterogeneity on the Crop Water Stress Index (CWSI). We present an assessment of the CWSI variability and its relationship with stomatal conductance (Gs) using different automatic object-based tree-crown detection algorithms based on temperature quartile thresholds. The study was carried out in an almond orchard cultivated under three irrigated regimes. High-resolution (25 cm) thermal imagery was acquired by an aircraft on summer 2015. The tree crowns were segmented into 4 classes using the 25th, 50th and 75th percentiles via automatic object-based methods. Results showed that CWSI was linearly and inversely correlated with Gs in all thermal classes. However, the relationship with Gs was heavily affected by the crown segmentation levels applied, and improved remarkably when CWSI values where those corresponding to the coldest and purest vegetation pixels (r2 =0.78 from pure vegetation pixels vs. r2=0.52 when warmer pixels were used).