Comparing the impacts of wildfire and meteorological variability on hydrological and erosion responses in a Mediterranean catchment

Land degradation and water resources pollution caused by catastrophic wildfires is of growing concern in fire-prone regions. Studies on the effects of wildfire on hydrology and erosion have mostly been conducted at plot or hillslope scale, while relatively few studies investigated post-wildfire hydrological responses and erosion at the meso-catchment scale (similar to > 10 km(2)) in the Mediterranean. This study used measured discharge and suspended sediment at the outlet of a burnt catchment in southern Portugal, before and after a wildfire, to investigate post-wildfire changes in hydrological and erosion responses to rainfall. Hydrological and sediment connectivity patterns were derived to investigate changing dynamics induced by the fire within the catchment. The main findings were: (a) although a large part of the catchment experienced moderate to high severity burning, post-wildfire hydro-sedimentary response was considerably limited; (b) meteorological variability determined hydrological responses and erosion more strongly than wildfire effects; and (c) during the post-wildfire vegetation recovery period, only rainfall events with a high return period (similar to 2 years) enhanced the hydrological and erosion responses. This can be explained by the spatial scale dependency of these processes and limited fine sediment supply, or relatively low connectivity in the study catchment. While connectivity is only an indicator, this implies that, at the meso-catchment scale, pollution of downstream water bodies by contaminated soil and ash may not occur immediately. Rather, because sediments and associated ashes and contaminants are first being transported to the areas around the stream networks, they only reach the outlet during heavy events which do connect the entire catchment. Thus, dynamic indices of connectivity that take rainfall event characteristics into consideration need to be further tested to assess and manage post-wildfire soil and water contamination risk.