Hillslope erosion and small watershed sediment yield before and after fire in southern California

Abstract

In 2002, a wildfire burned over an ongoing sediment flux study in the steep, chaparral-covered foothills of the San Gabriel Mountains of southern California. The study area had previously burned in 1960. Southern California experiences a Mediterranean climate with cool wet winters and hot dry summers. Average annual rainfall for the study area is 714 mm (80-year record) with an average two-year 30-minute peak intensity of 29 mm hr-1. Hillslope erosion was measured in 30 cm collector traps. These traps were serviced multiple times per year to distinguish wet season from dry season erosion. Small watershed (1-3 ha) sediment yield was measured behind earthen debris dams. Three of the study watersheds were in mixed chaparral vegetation and one was in type-converted grass. One of the chaparral watersheds was burned in a prescribed fire in 2001 and did not re-burn in the wildfire. Hillslope erosion and small watershed sediment yield data were collected for 7 or 8 years prior to burning then for 5 or 6 years following fire, including the complete post-fire erosion record. Continuous rainfall was measured in a centrally-located weighing bucket recording raingage. Rainfall, both totals and intensities, was generally below average during the study period, especially immediately after the fires. Prior to fire, hillslope erosion was considerable, was an order of magnitude less under grass vegetation compared to chaparral, dry season erosion was equal to wet season erosion in all watersheds, but sediment yield was only minor and associated with high rainfall years at the watershed scale. In the first post-fire year, both hillslope erosion and small watershed sediment yield increased by 1-2 orders of magnitude over pre-fire levels and all parts of the landscape responded similarly. Hillslope erosion rates and small watershed sediment yields returned to pre-fire levels and patterns within two to three years post-fire. The magnitude and patterns of erosion following a prescribe burn in chaparral was very similar to those following a wildfire, although the recovery curve was somewhat flatter after the prescribed fire. Future studies will test a variety of predictive models against the current dataset as a benchmark to evaluate their performance as a tool for planning and risk assessment in southern California chaparral watersheds.

Citation

Wohlgemuth, Peter M. 2015. Hillslope erosion and small watershed sediment yield before and after fire in southern California. Proceedings of the 3rd Joint Federal Interagency Conference on Sedimentation and Hydrologic Modeling, April 19-23, 2015, Reno, NV epublished, pp. 977-986.