Hydrological and Erosion Processes in the Research Catchments of Vallcebre (Pyrenees)

Abstract

The research catchments of Vallcebre were selected in 1988 for studying the hydrological and geomorphological consequences of Mediterranean middle mountain land abandonment and badland activity. The results obtained to date show that the hydrological response of old farmed areas is dominated by saturation mechanisms and yield very low sediment loads, because of the dense vegetation cover and the good condition of the old conservation structures (terraces and ditches). On the other hand, badland surfaces suffer very high erosion rates (about 9 mm per year), with a very dynamic behaviour regulated by physical weathering processes during winter and Hortonian overland flow and erosion when intense rainstorms occur. Catchments with significant badland area show a clear non-linear hydrological and sediment response. Characteristic unit hydrographs show the superposition of two peaks, the first one comes from degraded areas and is controlled by rainfall intensity, whereas the second and broader one comes from saturated areas and is controlled by the antecedent condition of the catchment. Suspended sediment concentrations measured during events occurring in dry periods are as high as 100 g l-1, but they fall by one order of magnitude for events during wet periods, because of the dilution with clear water contributed by the saturated stable areas.