Fire Severity on the Valley Complex, 2000

Wildfire is an important part of ecosystems and the pattern of vegetation that develops post-fire is highly dependent on the severity of the burn. The Composite Burn Index (CBI) is a standardized field measure of fire severity (Key and Benson, 1999) that is correlated with a satellite based severity measure, the Normalized Burn Ratio (NBR). The NBR utilizes the information contained in channel 4 (NIR) and channel 7 (MIR) of the Landsat Enhanced Thematic Mapper Plus (ETM+) satellite sensor. Calculated as the normalized difference of the reflectance in the two bands, the NBR takes advantage of the increase in MIR reflectance and the decrease in NIR reflectance post-fire. A differenced image (DNBR) can then be classified into four levels of severity (Low, Moderate-low, Moderate-high, and High). This study examines the performance of the DNBR logic on the Valley Complex fire of 2000 in the Bitterroot Valley, MT. A method for radiometric normalization and calculation of exo-atmospheric (at satellite) reflectance is given so that between-scene differences in sun angle and illumination can be accounted for prior to the change detection procedure. The physical setting comprising the study area is broken down into three terrain characterizing variables of aspect, slope class, and fire group. The proportions of each severity class that fall in each terrain variable class are discussed. It is found that the influence of aspect on severity distribution is due to its influence on vegetation, the influence of slope is due to its influence on fire residence time (i.e., the longer a fire remains in an area the higher the resulting severity), and that fire group provides the most insightful view into the severity distribution. Overall, the Valley Complex fires of 2000 did not burn in an extraordinary manner and the resulting severity was consistent with the vegetation type and physical setting through which the fire burned.

Principal Investigator: Steve Brown