Sediment and hydrological response to vegetation recovery following wildfire on hillslopes and the hollow of a small watershed

Wildfire can cause dynamically spatial and temporal variations of hydrology and sedimentation from forested environments. The largest wildfire historically recorded in South Korea burned 23,484 ha of forest land in April 2000. In 2001, systems to monitor post-fire runoff and sediment were installed at the outlet of a small watershed and at seven plots on hillslopes of different vegetation condition. The hillslope and catchment responses were monitored for five years post-fire. The watershed was divided into four sites including terrace-sodding (TS), pine-planting and fast recovery (PF), pine-planting and slow recovery (PS), and hollow (H) according to treatment methods and topographical characteristic. The TS site showed the fastest vegetation recovery among the sites and the greatest reduction of soil erosion with time elapsed after wildfire. Sediment yield from the watershed as well as from plots depended strongly on rainfall erosivity index. While the runoff coefficient showed the highest correlation with rainfall amount, the sediment response rate had the strongest correlation with a vegetation index characterizing vegetation structure, litter, and root. The hollow mostly acted as a depositional zone but only contributed to erosion when it was disturbed by the treatment applications following the fire or during heavy rain following landslide activity in the catchment. These results indicate that gently-sloping hollow areas with rapidly recovering vegetation have the potential to reduce the flood and sediment risk. However heavy and extreme rainfall events during the study produced excessive sediment by catastrophic debris flow and landslide processes even on vegetated hillslopes. The study suggests that the boundary condition for whether (i) rain is heavy or extreme, (ii) surface condition is bare, and (iii) terrain has a deposition zone should be considered in order to evaluate sediment yields from burnt catchments.