Relationship between temporal tendencies of sediment production-transport and climatic conditions in a small Mediterranean catchment (Vallcebre, Eastern Pyrenees)

Abstract

The Vallcebre catchment (Llobregat river head-waters) shows an important variety on land uses and geomorphic environments: forest, pasture, farmed and abandoned agricultural terraces, and badlands. Three nested catchments (Cal Rodó, Ca L'Isard and Can Vila) were equipped to measure hydrological and sedimentological yield from different land conditions. There was also set a raingauge net and two weather stations in the basin. Badland areas, that occur only in the Cal Rodó and Ca l'Isard catchments, are relatively small but very active, because both the abundant precipitation (924 mm year -1) and the high weathering rate in winter. Between September 1994 and December 1999 about 350 events were recorded at both the Cal Rodó and the Ca L'lsards station. At the Can Vila station some few tens of events were recorded, because it was not completely equipped until June 1998, in this case the badlands absence was reflected in a very poor sediment production. To study the temporal patterns on sediment transport a set of statistical analysis was performed, relating sedimentary parameters with the hydrological ones, at different temporal scales (single event, month, season and year). The results show that transport rates depend on the seasonal climatic conditions, directly by rainfall distribution, and indirectly by moisture and temperature annual regimes, because their great influence on the weathering badland dynamics. The most important conclusion related with the temporal scale suggested that results and their interpretation depend on the season or year taken into account. Mean erosion rates can be changed by a single event, because approximately a 90 % of the sediment yield is produced by only a 2 % of the events and a 17 % of the total precipitation. This means that, in these badland are as, precipitation distribution and characteristics (nor total amount), along wi.h badland weathering dynamics are the main factor controlling erosion rates and transport.