Hyperspectral detection of chemical vegetation stress: evaluation for the Canadian HERO satellite mission

The potential for direct detection of chemicals on vegetation and for detecting vegetation stress and damage from exposure was reviewed within the context of the proposed Hyperspectral Environment and Resource Observer ( HERO) satellite mission. The direct detection of 10 selected toxic chemicals was deemed unlikely. Detecting vegetation stress resulting from exposure to these chemicals may be feasible, based on several image analysis approaches considered for two levels of damage: (i) severe defoliation, mortality, and effects at the full crop or forest stand scale; and (ii) more subtle stress and damage such as discolouration, spotting, bleaching, and light defoliation that occur at subpixel scales. Image classification, vegetation indices, spectral mixture analysis (SMA), and change detection are expected to be suitable for the former, whereas only SMA and advanced modelling approaches are expected to be suitable for the latter because of the subtle nature of reflectance change at subpixel scales. A key recommendation is the need for a spectral library of toxic substances and the stress and damage caused to various types of vegetation at different growth stages. Current information on this is limited and represents a constraint to effective analysis and evaluation. Based on this application, HERO mission parameters were considered, with recommendations that the current spectral resolution and ground sampling distance (GSD) be improved, with a sensitivity analysis required to determine proper values to provide optimal detection of toxic hazards and vegetation stress. Other sensor and mission parameters such as spectral range, signal-to-noise ratio (SNR), swath width, and orbital repeat period with off-nadir sensing and temporal relook frequency were deemed acceptable, but improvements would be desirable. Extensive testing involving laboratory experiments and airborne hyperspectral imagery for a variety of toxic substances over different areas and vegetation types is required for assessment using different image analysis algorithms to confirm this and other recommendations for HERO prelaunch.