Assessing the co-occurrence of sheet and gully erosion in a wildfire-affected mountain area: A case study from Central Portugal

Soil erosion has been recognised as a major threat to soil resources. Wildfires have contributed to the acceleration of soil loss and degradation by inducing profound hydrological and geomorphological transformations, not only through intensified erosion, but also through changes in soil properties, vegetation cover and subsequent land use. This study evaluates two primary soil erosion processes, namely sheet and gully erosion. The objectives of this research are threefold: (i) to map sheet erosion using the InVEST Sediment Delivery Ratio (SDR) model; (ii) to identify areas where both sheet and gully erosion coincide; and (iii) to determine the dominant drivers of sheet and gully erosion. A combined methodology involving InVEST-SDR and ArcGIS was employed to assess erosion rates in the Alva basin, central Portugal, affected by 2017 wildfires. The presence of gullies aggravates the soil loss and confirms a high production of sediment resulting from sheet erosion and gully erosion in the study area. The results indicate soil loss in the study area ranging from 0 to 848.89 t. ha-1.yr-1, with an average in the study area of 1.29 t/year. In general, gullies are found in areas where sediment production is greater, on a regional scale, but not locally. The results confirm the importance of some coinciding factors for sheet and gully erosion, in particular flow accumulation (FA), slope, and land use cover (LUC). However, the slope is more decisive for sheet erosion and the LUC for gully erosion. Given the quantities of sediment produced and the size of the gullies, together with the recurrence of wildfires in the study area, some erosion control practices should be implemented as a matter of urgency, particularly after wildfires.