In the Pak and Lee method, the infiltration rate of the Surface layer after a fire is calculated as: This tutorial will implement the Dynamic Surface | Pak and Lee method. The Canopy, Surface, and Loss layers are conceptualized in the figure below. The burn severity of the Station Fire in the upper Arroyo Seco watershed is shown below (burn severity map accessed from Monitoring Trends in Burn Severity Direct Download page). Approximate ly 95% of the upper Arroyo Seco watershed and 63% of the Arroyo Seco watershed were burned. In August - October 2009, the Station Fire burned a large portion of the national forest.

The watershed has a drainage area of 16 square miles. The upper portion of the Arroyo Seco watershed (upstream of USGS 11152000 Arroyo Seco near Soledad, CA) will be modeled in this tutorial. The river joins the Los Angeles River north of downtown Los Angeles. The Arroyo Seco ('dry stream' in Spanish) originates in the Angeles National Forest and San Gabriel Mountains in southern California. This method can be used for post-wildfire or other scenarios where a surface layer of vegetation and/or soil limits the amount of precipitation that is able to infiltrate into the soil layer. Post-fire watershed modeling is important for estimating flood risk after fire and the rate of erosion, or recovery, of the hydrophobic surface layer. The Dynamic Surface method was introduced to HEC-HMS in version 4.11. Simulating watersheds in years immediately following a wildfire is difficult with constant soil infiltration rate. However, the recovery of soils can take years. The hydrophobic soil layer is eroded by rainfall. In addition, more rainfall is expected to runoff as surface and canopy storage is reduced, and evapotranspiration is minimized by the lack of trees and vegetation. The heat from a fire can bake the ground which creates a hydrophobic soil layer, increasing the speed with which water flows off the burn layer.

Wildfires dramatically change the ground conditions of a watershed, which can lead to increased flood risk. Natural, unburned vegetation and soil act as a sponge during a rainfall event. The final project files are provided at the end of the tutorial. Project Files Download the initial project files here:

Shapr3d tutorial pdf