Climate changes and the past 20 years of fire suppression practices have led to widespread wildfires in the western US. Wildfire can severely destroy the vegetation covers and changes the soil structure and alter the natural hydrologic, geomorphologic, and ecological processes. Postfire immediate disasters from burned watershed include flooding, erosion, landslide, and debris flow. To quicky estimate post-fire runoff and soil erosion, the method of curve number was applied through the modification of CN numbers for post-fire conditions. The method of CN number is a commonly used approach to calculate the surface runoff using the precipitation, initial abstract, and an empirical coefficient assigned to a watershed of specific vegetation cover and soil properties (NEH 2008). CN number ranges from 0 for surface waterbody that produces no runoff to 100 for impervious surface that 100% rainfall becomes runoff. The larger the CN number, the high runoff potential from a given watershed. CN values depend on the type and coverage densities of vegetation as well as soil properties. This study uses the latest vegetation data from LANDFIRE and SURROGO soil data developed the pre-fire CN maps for the Bighorn fire impacted watersheds in the Catalina mountain in Tucson, Arizona. The post-fire CN map was generated using the vegetation reduction map developed by the BEAR team. As vegetation density reduces, the areas of severe burns had an increase of CN value as large as 14, while areas of minor burns had only slight or no increase of CN value. The mapped CN maps were validated by the simulation of hydrological processes using HEC-HMS model for a few observed pre- and post-fire rainfall and runoff events. The results showed the CN method is an accurate and effective way to predict post-fire surface runoff for emergence management.
