Post-fire watershed response at the wildland-urban interface, southern California

Abstract

In southern California, the unrelenting urban expansion into neighboring uplands has created a wildland-urban interface that is increasingly difficult to manage. In September 2002, the Williams Fire burned the San Dimas Experimental Forest (SDEF), mostly at high severity. This event provided an opportunity to describe and analyze the impacts of fire and the historical management practice of type-conversion on post-fire runoff, sediment yield, soil water repellency, and vegetation recovery in chaparral ecosystems at the wildland-urban interface. Results indicate that soil water repellency increased with depth, declined with time since fire, was inversely related to soil moisture, and was only slightly different with the two pre-fire vegetation types. Herbaceous grasses and forbs dominated the post-fire vegetation initially, but all watersheds are reverting back to their pre-fire plant communities. Bare ground declined with time since fire, primarily as the litter layer accumulated. Number of species per watershed was similar with the two pre-fire vegetation types, although the species composition was different. Comparisons revealed similar magnitudes of post-fire watershed response for both pre-fire vegetation types. Runoff was large in the first post-fire year despite only moderate rainfall. Runoff exceeded the measurement equipment during the record rainfall year of 2005. Sediment yield was large immediately after the fire but was negligible the following year. However, sediment yield was minor during the record rainfall year of 2005, suggesting that the supply of easily mobilized debris was depleted after the first post-fire winter. If there were any differences in fire behavior between the two vegetation types, the landscape exhibited nearly identical fire effects and watershed responses. Summarizing results and their applications in an effective format remains the greatest challenge in communicating science to policy- and decisionmakers. For this project, the traditional technology transfer tools of written reports and symposia presentations were supplemented with field tours and a special workshop to which all local Federal, State, county, and municipal land mangers, hazard protection agencies, and political administrators were invited.

Citation

Wohlgemuth, Peter M.; Hubbert, Ken R.; Beyers, Jan L.; Narog, Marcia G. 2008. Post-fire watershed response at the wildland-urban interface, southern California. In: Webb, R.M.T., and Semmens, D.J., eds. Proceedings of the Third Interagency Conference on Research in the Watersheds: U.S. Geological Survey Scientific Investigations Report 2009-5049: 137-142.