Hec-Hms Gridded Cn Method For Post Fire Hydrology

In the past several years, very large wildfires have become increasingly common. These wildfires may result in short-term impairments (1 – 5 years) to watersheds which can create dramatic increased threats from post-fire flooding and debris flows. Because of this, rapid assessments are needed to accurately identify potential threats to life and/or property. Post-wildfire watershed modeling is a key component in these emergency assessments by providing estimated increase in runoff and data for flood inundation boundaries. Using the Distributed CN method is essential for obtaining reliable results of post-fire CN based models. A post-fire modeling procedure has recently developed by the USDA Natural Resources Conservation Service, in cooperation with the US Army Corps of Engineers Hydrology and Statistics Division, that utilizes the Gridded CN and Modified Clark Transformation methods (Gridded CN/Mod Clark) which are available with the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) software. The Gridded CN/Mod Clark procedure is similar to using the Distributed CN method with the added benefit of calculating runoff spatially by user defined geospatial grids. This procedure can provide quicker and more reliable model results from post-wildfire affects over other CN based models. This is especially true when modeling effects from large fires that require hydrologic routing.