All the catchments are laying on the same geological bedrock: Leucogranodiorite of Plomelin. The upper part of this granite is weathered on 1 to more than 20 m deep so that it cannot be considered as impervious. Part of the flow is likely to percolate deep through the fractures into the bedrock. The soils are mainly sandy loam. The superficial layer (0-20 cm) is rich in organic matter (4.5 to 6%). The average soil depth is 0.80m. Soils have been mapped at the scale of the site, they are well drained except in bottomlands close to the streams.