Flood generation and sediment transport in experimental catchments affected by land use changes in the central Pyrenees

Summary Three small catchments ( 2 in size) were monitored in the Central Spanish Pyrenees to analyse the hydrological and geomorphological consequences of different land covers under the same climate scenario: (i) the San Salvador catchment represents a dense, undisturbed forest environment; (ii) the Arnas catchment corresponds to an old, abandoned cultivated area subjected to colonisation by plants; and (iii) the Araguas catchment consists in part of active badlands. The obtained results demonstrate that plant cover is a key factor, influencing (i) the seasonality and intensity of floods, (ii) the annual volume of discharge, and (iii) the suspended sediment concentration, total sediment yield and proportions of different types of sediment. The forested catchment tends to generate floods in late winter and spring, when the water table is close to the surface, and flood hydrographs are generally gentle, with solutes largely prevailing over suspended sediment. The old agricultural catchment produces in average twice the number of floods as that recorded in the forested catchment, with the highest floods recorded in autumn and spring; this catchment behaves as a complex mosaic, with a large spatial and temporal variability in terms of both sediment- and runoff-contributing areas; in addition, suspended sediment is equal to solutes, and bedload reaches a relatively high importance. Finally, the badland catchment reacts to all rainstorm events throughout the year, with a high suspended-sediment load. Sediment outputs from the Araguas catchment are two orders of magnitude higher than in the Arnas and San Salvador catchments. Suspended sediment concentrations exceed 300 g l −1 in the Araguas catchment, whereas they rarely exceed 20 g l −1 in the Arnas and rarely 1.5 g l −1 in the San Salvador catchment.