Modelling forest fire and firebreak scenarios in a mediterranean mountainous catchment: Impacts on sediment loads

Abstract

Forests provide a number of ecological and hydrological services, for instance, contributing to decreased water and sediment yields through increased infiltration and reduced soil erosion. However, forest fires can turn positive forest services into drawbacks, enhancing surface runoff and soil erosion and damaging both hillslopes and downstream aquatic life in rivers. Therefore, appropriate mitigation strategies should be developed to limit these negative effects. Using a runoff and erosion model (the WaterSed model), we proposed forest fire and firebreak scenarios to analyse their respective effects on sediment loads. The model reproduced the measured discharge and sediment loads over an entire hydrological year, including 21 flood events occurring from November 2010 to May 2011 in a 72-km² Mediterranean catchment (Celone catchment, Puglia, Italy). Eight different forest fire scenarios were then proposed. While the mean burnt areas remained below 2% of the total catchment area, forest fires significantly affected the sediment yield. Indeed, the sediment yield increased over the different forest fire scenarios, from 1.97 to 2.70 t ha^-1.yr^-1, corresponding to a 37% increase. At the flood-event scale, the sediment load after fire represented up to 324% of the unburnt catchment sediment load in the worst-case scenario. By using realistic firebreaks, the sediment load could be dramatically reduced, from 324% to 165%, in the worst-case scenario. Because rural catchments, such as the Celone catchment, are currently experiencing land abandonment, forested areas are expected to replace crops and expand in the future. This change will likely increase forest ecological services, which may, however, be punctually balanced by negative fire effects. More studies addressing the global impacts of forest growth, fires and firebreaks on sediment transfers are therefore needed in similar environments.

Keywords: Firebreaks; Flood events; Forest fires; Runoff; Soil erosion.