Abstract
In 2002, the Williams Fire burned >90 % of the San Dimas Experimental Forest, providing an opportunity to investigate differences in soil water repellency, peak discharge, and sediment yield between grass-converted and chaparral watersheds. Post-fire water repellency and moisture content were measured in the winter and summer for four years. Peak discharge was determined using trapezoidal flumes with automated stage-height recorders. Sediment yields were measured by making repeated sag-tape surveys of small debris basins. Other than the high summer 2005 increase in repellency on the grass watersheds, only small differences in repellency were observed between the grass and chaparral sites. In general, soil water repellency increased with depth, decreased with time following the fire, and was inversely related to soil moisture content (i.e., least repellent during the winter and most repellent during the summer). Reduction in repellency occurred at moisture contents ranging between 8 % to 16 %. Approximately 85 % of the sediment delivered to the debris basins occurred during the first year, with first year sediment yields being greatest in the chaparral watersheds. Peak discharge was similar for both the grass and chaparral watersheds and was highest following the record rainfall of the 2005 hydrologic year. However, only minor sedimentation followed the record rain events and was similar in both watershed types, suggesting that percent plant cover was sufficient and that the supply of easily mobilized sediment and ravel was depleted after the first post-fire winter.
Highlights
Wildfires commonly occur on shrubland ecosystems linked with Mediterranean climates throughout the world
In southern California as well as other similar regions, there is usually a first year peak of sediment following wildfire, which can remain elevated for 3 yr to 10 yr (Rowe et al 1954, Florsheim et al 1991, Wohlgemuth et al 1999), with sediment yield amounts decreasing as vegetation becomes reestablished (Barro and Conard 1991)
The objectives were to: 1) assess whether type conversion of chaparral to grass would change the persistence of repellency on the landscape; 2) assess how vegetation recovery would affect hydrologic response between the grass and chaparral sites; and 3) compare temporal changes in sediment yield between the grass and chaparral watersheds
Summary
Post-fire soil water rePellency, hydrologic resPonse, and sediment yield comPared between grass-converted and chaParral watersheds. The primary objectives of this study were to compare post-fire changes in soil water repellency, vegetation cover, hydrologic response, and sediment yield between mixed chaparral and type-converted grass watersheds over a four year period. Accelerated post-fire runoff and erosion following wildfire can overwhelm the ephemeral and intermittent stream channels in the headwater tributaries, scouring channels and generating floods and debris flows, resulting in the loss of life, property, and structures located in and around natural debris basins and drainages (Munns 1920, Kraebel and Sinclair 1940, Wells 1987) In this regard, results from this study will aid federal, state, county, and municipal land and watershed managers worldwide who must be able to quantitatively predict the effects of post-fire management actions on the hydrologic response and subsequent sediment yield for shrubland watersheds
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