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

Abstract

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 (~ > 10 km2) 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 (~ 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.

Impacts of wildfire and post‐fire land management on hydrological and sediment processes in a humid Mediterranean headwater catchment

“…Ferreira et al (2008) proposed a decrease in erosion rates from hillslopes to catchments in recently burnt areas due to a decrease in sediment connectivity caused by a limited sediment transport capacity of streamflow. Lower connectivity at the catchment scale was also reported for several Mediterranean burnt catchments (Inbar, Tamir, & Wittenberg, 1998;Keizer et al, 2015;Lavabre & Martin, 1997;López-Vicente et al, 2020;Wu et al, 2020). In Macieira, however, most of the sediments eroded by hillslope runoff processes appeared to have left the catchment within the first two years after the fire, although with a time lag between rill erosion and sediment yield (Figure 3d).…”

“…Streamflow and sediment yield of individual events were mainly determined by post‐fire rainfall conditions, with fire occurrence playing a secondary role at most as observed elsewhere (Mayor et al, 2007; Stoof et al, 2012; Wu et al, 2020). While streamflow did not reveal any obvious impacts of the present fire, sediment yields did.…”

“…However, while a larger fire might lead to a larger sediment supply, it would not necessarily lead to higher sediment concentration due to lower in-stream connectivity caused by transport capacity limitations. The additional sediments (including wildfire ash) might accumulate in lower slopes and the stream bed, and be exported in subsequent years, as has been observed earlier in burnt catchments (Inbar et al, 1998;Mayor et al, 2007); or even deposit in the floodplain for larger watersheds (Wu et al, 2020). Further studies in catchments burnt to a greater extend are urgently needed to assess this.…”

“…Such an event‐based analysis has also been successfully used to understand hydrological and sediment processes in burnt headwater catchments (Brogan, Nelson, & MacDonald, 2019; Mayor, Bautista, Llovet, & Bellot, 2007; Moody et al, 2013). However, in the Mediterranean region only a few catchment‐scale studies comparing burnt and unburnt conditions have been conducted (Lavabre & Martin, 1997; Lavabre, Torres, & Cernesson, 1993; Mayor et al, 2007; Stoof et al, 2012; Wu, Baartman, & Nunes, 2020). Studies that compare burnt forest catchments with pre‐afforestation agricultural catchments have not been published so far, hampering the understanding of the hydrological and erosion impacts of afforestation (and subsequent forest fires).…”

Impacts of wildfire and post-fire land management on hydrological and sediment processes in a humid Mediterranean headwater catchment

“…Such an event-based analysis has also been successfully used to understand hydrological and sediment processes in burnt headwater catchments (Mayor et al, 2007;Moody et al, 2013;Brogan et al, 2019). However, in the Mediterranean region only a few catchment-scale studies comparing burnt and unburnt conditions have been conducted (Lavabre et al, 1993;Lavabre and Martin, 1997;Mayor et al, 2007;Stoof et al, 2012;Wu et al, 2020). Studies that compare burnt forest catchments with pre-afforestation agricultural catchments have not been published so far, hampering the understanding of the hydrological and erosion impacts of afforestation (and subsequent forest fires).…”