Long-term erosion and the impact of wildfires: two different approaches

Abstract

Rivers act as a network of channels that carry water, sediment, and energy downstream. On the heterogeneous area of the watershed, some areas might be responsive to disturbance (e.g., wildfires and droughts), while others may be resistant to the change. As a disturbance, fire is usually regarded as a key agent of soil erosion and land degradation. Fires are thought to be responsible for: (i) overland flow and reduction of the capacity of infiltration; (ii) increase of the availability of ash and debris and disruption of the soil nutrient cycle; and (iv) increase of connectivity across the watershed. The potential fire effects on soils and aquatic resources have created a strong demand for a post-fire sediment loss prediction tool. Taking this in mind, this study aims to assess post-fire soil erosion patterns at the decadal scale comparing different approaches. The methodology comprises i) a process-based model that is able to investigate long-term and large-scale spatial landscape evolution, LAPSUS; (ii) an index that represents a connectivity assessment based on local landscape information, the Borselli Index of Connectivity (IC); and (iii) an index that represents the sediment eroded that actually reaches the stream based on local landscape information, combining the IC with the Revised Universal Soil Loss Equation (RUSLE) model. Results include a comparison between the approaches used in the context of specific fire events between 1979 and 2020 for the Agueda watershed in central Portugal. The authors believe that assessing the spatial-temporal evolution of connectivity in the actual landscape with the appropriate tool is extremely important to estimate the probability that a given part of the landscape transfers its sediments downstream.