A susceptibility model for post wildfire soil erosion in a temperate oceanic mountain area of Spain

Abstract

In the North-West of the Cantabrian Range (North of Spain) the climate is oceanic and vegetation cover is continuous. In those areas where livestock farming prevails, wildfires are common, although small in size, their recurrence makes the phenomenon critical for the conservation of soils. In this study we propose that the structural stability of soil, associated with the type and size of the structural aggregates, may be a useful indicator to assess erosion susceptibility in burnt soils. We have chosen an area of approximately 485 km 2 over quartzitic lithologies where a high recurrence of wildfires has been noted and which displays several forms of erosion: gullying, rilling and erosion by overland flow. We have measured texture, percentage and degree of structural aggregate stability and the rate of infiltration in soils that have been burnt up to 3 times over the last 20 years and also in unburnt soils. The results obtained enable us to establish connections between wildfires, soil deterioration and macro-aggregate stability. We have used the stability of macro-aggregates as an indicator to elaborate a soil erosion susceptibility model for a large area of 10,600 km 2 with sharp relief and Atlantic climate. The model was constructed by combining three main factors: soil structural stability, fire intensity and relief. Variables related to soil structural stability and presence of basic cations have been derived from lithology (% Clay and % Silt + Fine Sand). The availability of humified organic material has also been taken into account as an additional variable in the formation of stable macro-aggregates. The expected fire intensity was calculated from the amount of inflammable material and the structure of the different vegetation types. Finally, the influence of relief was analyzed by considering the slope steepness. The resultant cartographic model presents five types of post wildfire soil erosion susceptibility. Those areas with the highest risk correspond to quartzite lithology regions, with long, steep hillsides covered with heaths. Those with the lowest risk correspond to limestone bedrock areas with gentle slopes and herbaceous vegetation. The accuracy of the model is determined by the scale of the original thematic cartography: 1:25000, and the cell size of the Digital Terrain Model is 50 × 50 m.