Thermal‐water stress index from satellite images

Forest fires constitute a major disturbance factor in the North‐American landscape. To evaluate the thermal and water stress of the vegetation canopy in Southern Québec as a test area, leaf water status was evaluated from vegetation indices derived from SPOT VEGETATION (VGT) images and surface temperature from Advanced Very High Resolution Radiometer (AVHRR) images. This study was conducted by investigating the vegetation conditions for two different periods, from June to August, 1999 and 2000. The Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Water Index (NDWI) were derived from SPOT VGT for evaluating vegetation condition and water status. For thermal status, surface temperature was retrieved by the split‐window method from AVHRR. The two vegetation indices were integrated for evaluating the vegetation condition and water status as a new index, the Normalized Moisture Index (NMI). The NMI was negatively correlated with surface temperature. A theoretical trapezoid in spectral–temperature space was defined by the NMI and surface temperature, and the thermal and water status of the vegetation canopy was determined according to separate small sections within the trapezoid. The thermal and water status derived from the trapezoid was validated by comparison with the indices of the Canadian forest Fire Weather Index (FWI) system.