Modelling the effects of land-use change and climate change on runoff and sediment yield for the Ribera Salada in the Southern Pyrenees

Introduction Substantial parts of the Pre-Pyrenees in the north-eastern part of Spain have experienced an extensive land-use change over the second half of the 20th century owing to the reduction of agricultural activities towards the formation of a more natural landscape (Lasanta-Martinez et al. 2005, Garcia-Ruiz et al. 1996). Traditionally, the area was characterised by intensive agricultural use even on very steep hillslopes that led to severe soil erosion. Due to a demographic change and socio-economic change, the migration of the rural population and consequent depopulation of the region (Ortigosa et al. 1990, Garcia-Ruiz and Valero-Garces 1998), many cultivated fields were abandoned, especially the terraced fields on the slopes while in the valley bottoms agriculture has been intensified. Abandoned fields have either been affected by a natural process of plant re-colonisation by mostly shrubs, or have been reforested with conifers. The fast abandonment of traditional agricultural practices and the large extent of the areas affected by vegetation recovery make the Pyrenees a good example for assessing the impact of land-use change on the hydrological response and sediment delivery dynamics (Gallart and Llorens 2004, Lopez-Moreno et al. 2006). Afforestation is reported to have changed the hydrological behaviour of headwater catchments of the Ebro basin by modifying surface runoff and by reducing the peak flow and by reducing soil erosion and sediment export. According to Garcia-Ruiz et al. (1996) and Ortigosa and Garcia-Ruiz (2000), some of the most important rivers in the region and their alluvial fans have recently stabilised their sedimentary structures, caused by plant re-establishment in the channels and on the riverbanks. Lopez-Moreno et al. (2006) analysed the evolution of floods in the central Spanish Pyrenees during the period 1955-1995 and detected a general negative trend in flood intensity, an increase in the importance of low flows in the total annual contribution against a stable frequency distribution of precipitation events. They linked of a decrease in the siltation rates in the Pyrenean reservoirs (with a consequential increase in their expected life-span) with a decrease in the torrentiality of Pyrenean rivers. Land-use and plant-cover changes were estimated to be responsible for the loss of up to 30 % of the average annual discharge from the 1950s until present (Begueria et al. 2003). During the so-called traditional land-use system, soil erosion was probably the most important environmental problem in the region (Garcia-Ruiz et al. 1996). At present, at least one third of the former agricultural areas remain bare, i.e. without dense shrub or forest cover, hence the observed negative trend of water and sediment fluxes may continue in the near future if vegetation recovery continues. On the other hand, the region may experience a renewed increase of agricultural production as, for example, vineyard plantations are forced to move up from the Ebro plains to the foothills of the Pyrenees because conditions in their traditional vineyards become increasingly dry due to the regional effects of global warming (Nash 2007).