POST-FIRE WATER YIELD, SEDIMENT YIELD, AND LOG EROSION BARRIER EFFECTIVENESS IN SMALL FORESTED WATERSHEDS, SOUTHERN CALIFORNIA

A high severity wildfire in mixed pine/oak forest on granitic terrain burned a series of headwater catchments in the San Jacinto Mountains near Idyllwild, California in the summer of 1999. Log erosion barriers (LEBs) were constructed across much of the burned area as a post-fire hillslope erosion control measure. To quantify post-fire water and sediment yield and to evaluate the effectiveness of the LEBs, we installed debris basins in two 1-hectare watersheds—one treated with LEBs and one untreated. The study area received 300 mm of precipitation in the first post-fire year and 370 mm during the second year post-fire. About 80 percent of this precipitation fell as rain and snow in low-intensity winter cyclonic storms, while the remaining 20 percent came in high-intensity summer thunderstorms. The degree of fire-induced soil water-repellency, measured by timed waterdrop infiltration, was similar over the two catchments; while soil depths, determined by augering, proved to be nearly twice as great in the watershed without the LEBs compared to the treated catchment. Water yield, based on pond surveys, was greater in the treated watershed in the initial post-fire year, but this comparative response equalized or reversed during the following year. Surveys revealed that the 157 LEBs retained about 4 m3 of sediment in the first year after the fire and an additional 9 m3 of material the next year. Sediment yield, based on the water-corrected weight of debris excavated from the two reservoirs, was 14 times greater from the treated watershed in the first year post-fire, but was 18 times greater from the untreated catchment during the following year. Water yield is primarily generated by the winter cyclonic storms, while it appears that the spatial and temporal disparities in water yield are governed by differences in soil depth and antecedent soil moisture. Sediment yield is produced primarily by the summer thunderstorms, with the spatial and temporal patterns reflecting differences in runoff and protective ground cover. Although the LEBs retained considerable amounts of sediment, the impact of this management treatment was masked by differences in inherent site characteristics.