Predicting fire in Yosemite National Park: Snow depth, natural ignitions, burn severity, and burn complexity

Yosemite National Park has experienced 1871 fires between 1984 and 2005 totaling 76,558 ha in extent. Each of 104 fires > 40 ha in extent (73,264 ha; 95.7% of total area burned) was analyzed using multi-temporal satellite imagery (pre-fire and post-fire). The Relative Normalized Burn Ratio (RdNBR) applied to Landsat images was used to stratify fires into patches burned at low, moderate and high severities. Within each fire, patches burned at each severity were analyzed using the simple proportion of area burned at each severity and the landscape metrics patches per unit area (PPU) and squareness of patches (SqP). Within Yosemite National Park between 1984 and 2005, there was no overall temporal trend in area burned. However, the area of the largest fire in each year increased. The number of lightning ignitions (natural fires) within Yosemite National Park was strongly related to the April 1st snow water content in the Tuolumne River watershed, potentially providing a useful a priori predictor for the fire season. Within the park boundaries, the area burned by lightning ignited fires was related to April 1st snow water content. The size of high severity and moderate severity patches and the proportion burned at high severity and moderate severity were related to the total annual area burned and also to April 1st snow water content. Burn patch complexity was only weakly related to April 1st snow water content. Instead, burn patch complexity was strongly related to the total area burned.