Post-fire erosion and sediment yield in a Mediterranean forest catchment in Italy

Wildfires are an increasingly alarming phenomenon that affects forests and agroecosystems, generating several cascade effects among which soil erosion is one of the most deleterious. A robust body of data-based evidence on post-fire soil erosion and sediment yield at the watershed scale is, thus, required, especially when dealing with areas where wildfires are particularly frequent, such as the Mediterranean basin. This study analyzes the impact of the first rains after a large wildfire in terms of soil erosion and sediment yield at the watershed scale in a Mediterranean area, the Pisan Mountains, central Italy. Here about 1,000 ha of olive groves, maquis, maritime pine, and chestnut forests, all on steep slopes, burned in 2018. Fire (or burn) severity was mapped by remote sensing and checked by a field survey. Sediment yield was assessed by sampling earthy materials deposited upstream of a check dam at the outlet of the studied watershed. Finally, a hydrological model was developed in the hydrologic engineering center–hydrological modelling system (HEC–HMS) environment to explore the relationship between the erosion–deposition events observed in the watershed and the rainfall-induced hydrological processes. The first two post-fire rainy events relocated a high mass of sediment, mostly non-organic and characterized by light color, perhaps already in the stream before fire, while the subsequent four rain showers deposited materials rich in pyrogenic organic matter. Overall, the soil erosion caused by these six major rainfall events–the larger of which had a return time of one year–was estimated to amount to 7.85 t/ha (0.26 mm in the watershed), corresponding to 42% of the watershed average annual potential erosion rate in unburned conditions. This value is lower than expected, and, overall, moderate if compared to other Mediterranean case studies, possibly because of the nature of soils in the watershed, i.e., shallow and stony, thus, poor in fines prone to erosion.