Assessing canopy PRI for water stress detection with diurnal airborne imagery

A series of diurnal airborne campaigns were conducted over an orchard field to assess the canopy Photochemical Reflectance Index (PRI) as an indicator of water stress. Airborne campaigns over two years were conducted with the Airborne Hyperspectral Scanner (AHS) over an orchard field to investigate changes in PRI, in the Transformed Chlorophyll Absorption in Reflectance Index (TCARI) normalized by the Optimized Soil-Adjusted Vegetation Index (OSAVI) (TCARI/OSAVI), and in the Normalized Difference Vegetation Index (NDVI) as function of field-measured physiological indicators of water stress, such as stomatal conductance, stem water potential, steady-state fluorescence, and crown temperature. The AHS sensor was flown at three times on each 2004 and 2005 years, collecting 2 m spatial resolution imagery in 80 spectral bands in the 0.43–12.5 μm spectral range. Indices PRI, TCARI/OSAVI, and NDVI were calculated from reflectance bands, and thermal bands were assessed for the retrieval of land surface temperature, separating pure crowns from shadows and sunlit soil pixels. The Photochemical Reflectance Index, originally developed for xanthophyll cycle pigment change detection was calculated to assess its relationship with water stress at a canopy level, and more important, to assess canopy structural and viewing geometry effects for water stress detection in diurnal airborne experiments. The FLIGHT 3D canopy reflectance model was used to simulate the bi-directional reflectance changes as function of the viewing geometry, background and canopy structure. This manuscript demonstrates that the airborne-level PRI index is sensitive to the de-epoxidation of the xanthophyll pigment cycle caused by water stress levels, but affected diurnally by the confounding effects of BRDF. Among the three vegetation indices calculated, only airborne PRI demonstrated sensitivity to diurnal changes in physiological indicators of water stress, such as canopy temperature minus air temperature (Tc–Ta), stomatal conductance (G), and stem water potential (ψ) measured in the field at the time of each image acquisition. No relationships were found from the diurnal experiments between NDVI and TCARI/OSAVI with the tree-measured physiological measures. FLIGHT model simulations of PRI demonstrated that PRI is highly affected by the canopy structure and background.