Runoff and Sediment Yield from Land under Various Uses in a Mediterranean Mountain Area. Long-term Results from an Experimental Station

In this study we analyzed runoff and sediment yield from land under various traditional and current land uses in Mediterranean mountain areas, using long-term data from an experimental station in the Aísa Valley, Central Spanish Pyrenees. The Aísa Valley Experimental Station was established in 1991. The operation and monitoring at this station has provided 20 years of data that can help explain the hydrological and geomorphological changes that have been observed at larger spatial scales, and also the changes that have occurred to some of the most characteristic landscapes of the Mediterranean middle mountains. In spite of the problems associated with the use of small experimental plots (including those related to the lack of connection between the plot and the adjacent hillslope, disturbances introduced by the limits of the plot, and sediment exhaustion), the results obtained are consistent with other studies in the Mediterranean region, and confirm the strong influence of land cover/land use changes on runoff generation and sediment yield. The results indicate that: (i) cereal cultivation on steep slopes (both alternating of cereal cultivation and fallow on sloping fields and shifting agriculture on the steepest slopes) represents a major problem for soil conservation. This explains the occurrence throughout the Mediterranean mountains of many degrades hillslopes, which have thin and stony soils and show evidence of sheet wash erosion, rilling, gullying and shallow landsliding; (ii) farmland abandonment has led to a marked reduction in runoff and sediment yield as a consequence of rapid plant recolonization, particularly where the plant cover is dominated by dense shrubs; (iii) the natural transformation of abandoned fields into grazing meadows has reduced runoff and sediment yield, increasing the availability of grasslands for livestock. Land use trends in the Mediterranean mountains are mainly characterized by generalized farmland abandonment and a decrease in the livestock pressure throughout most of the region. From a hydrological and geomorphological point of view the main consequences have been a reduction in overland flow from the hillslopes, and a reduction in sediment sources, with differences up to one order of magnitude in sediment yield from dense shrub cover and grazing meadow areas compared with areas under shifting agriculture.