Small-scale portable rainfall simulators are an essential research tool for investigating the process dynamics of soil erosion and surface hydrology. There is no standardisation of rainfall simulation and such rainfall simulators differ in design, rainfall intensities, rain spectra and research questions, which impede drawing a meaningful comparison between results. Nevertheless, these data become progressively important for soil erosion assessment and therefore, the basis for decision-makers in application-oriented erosion protection.

The Vallcebre catchments are located in a middle mountain area of the Pyrenean ranges, built up by sedimentary rocks and loamy soils. The vegetation cover is pastures and forests of Pinus sylvestris, mostly occupying former agricultural terraces. Some relatively small, heavily eroded landscapes (badlands) occur in the catchments, playing a relevant hydrological and geomorphic role. Annual precipitation is 924 mm and potential (reference) evapotranspiration is about 700 mm. Rainfall interception in forests represents about 24% of precipitation; interception rates were similar throughout the seasons because of a compensation between rainfall intensities and atmospheric conditions. Soil moisture showed a temporal pattern characterised by the occurrence of marked deficit periods in summer and also, but less pronounced, in winter. During most of the year, subsurface flows on hillslopes drove the spatial organisation of soil moisture and the occurrence of saturated areas. Nevertheless, this spatial organisation was also controlled by the patterns of vegetation cover. During dry periods, subsurface flow ceased, saturated areas disappeared and the spatial patterns of soil moisture changed. Stream flow from these catchments was dominated by storm flow, and the runoff generating mechanisms showed a clear seasonal pattern, controlled mainly by the soil moisture and the extent of saturated areas. During the dry periods, runoff was produced only on impervious areas and badlands. At the end of the dry periods, some large rainfall events generated significant runoff because of the perched saturation of the shallow soil horizons. Thereafter, runoff generation was dominated by the role of saturated areas. Stream waters in catchments with badlands had very high suspended sediment concentrations. The seasonal pattern of erosion processes in badlands was characterised by physical weathering during winter, regolith breakdown and vigorous hillslope erosion during spring and summer, and efficient transport of sediments in autumn.

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The genesis and development of badlands is controlled by bedrock lithology, topography and climate. The development of badlands in the Central Spanish Pyrenees is favoured by the presence of marls and a climate with a marked seasonality; the combination of these factors results in active hydrosedimentological processes. The aim of the present work was to study the dynamics of badlands in the Central Pyrenees, with a focus on the temporal and spatial patterns of the dominant weathering processes. The study area was a small mountain catchment, and we focussed on the continuous observation of a number of physical parameters that control regolith development and weathering processes. Samples collected from different depths at sites with different aspects enabled temporal comparisons of the factors involved in the development of surface regolith that can be considered as indicators of the weathering of the marls. These indicators were then related to environmental variables (moisture and temperature). Atterberg limits were established and related to moisture content to explain slope dynamics and related processes. Finally, analyses of carbonate content were performed in the laboratory. The results validate the employed methodology, supporting our description of weathering processes, the established chronological sequence and the relation of these processes to climatic and topographic parameters. The results obtained since January 2004 show that the chosen indicators are sensitive to seasonal dynamics, variations in temperature and water supply, and slope aspect. The development and dynamics of regolith on north-facing slopes is more active than that on south-facing slopes. Freezethaw is the main weathering process on north-facing slopes, while south-facing slopes are dominated by the development of crusts associated with wetting-drying processes. Washing effects and lower temperatures upon north-facing slopes mean that the dissolution of carbonates is more pronounced in such areas compared with south-facing slopes, especially within the upper layers of the regolith. favoured by lithology, topography and a climate that is characterized by strong seasonal contrasts in temperature and rainfall distribution. Badlands are characterized by features such as the absence of vegetation, steep slopes and a highdensity drainage network, and hence tend to be among the most significant areas of erosion in the world. This fact is demonstrated by high concentrations of sediment in such areas, as in the Spanish Pyrenees (Valero-Garcés et al., 1999;Beguería, 2005) and the French Alps ('Terres noires'), which can exceed 1000 g l −1 (Descroix and Mathys, 2003). Badlands developed in sub-humid mountain areas are subjected to higher rates of denudation and more active dynamics compared with similar landscapes in arid or semiarid areas (Gallart et al., 2001;Regüés and Gallart, 2004). Rapid and deep weathering (Schumm, 1956), together with intense soil erosion, explain the occurrence of very high erosion rates in badland areas; in some cases...

This paper studies the hydrological response to rainstorm events of a small experimental catchment in the Central Spanish Pyrenees. The Arnás catchment was cultivated until 40 years ago, and then abandoned and affected by plant recolonisation, especially shrubs. A rainfall of a few mm is enough to produce a sudden increase in discharge, due most probably to the steep gradients and the small size of the catchment and the extensive areas with low vegetation density and thin soils. The intensity of the response shows a very high variability, depending on the intensity of precipitation and soil humidity conditions before the flood. This paper identifies two types of floods according to the relationships between precipitation and discharge, and confirms that antecedent soil moisture explains much of the response. The shape of the hydrograph, very similar to the hyetograph, suggests that the Arnás catchment is dominated by overland flow processes. However, more intense rainstorms do not generate higher peak flows, thus demonstrating the existence of different runoff generating areas.

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