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The action of the Quaternary glaciers in the Central-Western Pyrenees is responsible for the various kinds of the relief features found there, including the well-configured cirques, large U-shaped valleys, overexcavation basins, detailed shapes, and moraine accumulations of very different ages. With almost continuous divides exceeding 2500 m and very complex basins that integrate numerous tributary valleys, the main Pyrenean glaciers reached over 30 km in length and 400 m in thickness, descending to altitudes of c. 800 m. The largest glaciers were those in the Gallego and Esera valleys. Their size is attributed to the large extents of their headwaters and the altitudes of their highest peaks. The long tradition of studies on Pyrenean glacial processes allows us to trace a fairly complete sequence of events, especially during the Last Glacial Cycle. Thus, a considerable amount of data confirm that the maximum ice extent did not occur during the global Last Glacial Maximum (20 ka or MIS-2) but at 60 ka, that is, during MIS-4. There is indirect evidence of a glacial advance between 22 and 19 ka as well as successive advances and retreats in the Oldest Dryas and Younger Dryas during deglaciation. Moreover, small reactivations likely took place during the Holocene from 6 ka, especially during the Little Ice Age. It is also interesting to note that scattered moraine remains, erratic blocks, and fluvioglacial terraces corresponding to previous glacial cycles have been found in some valleys.

1 Pág. The original version is available at: https://www.sciencedirect.com/science/journal/0169555X, Regarding the contents of the Supplementary material 2, which contained the erosion rates database used in the article, we regret that the plain text file was corrupted and some of the erosion rates are wrongly formatted. We would like to apologise for any inconvenience caused. We provide the complete database in Microsoft Excel format. This file should be used instead of the original one.

46 Pags.- 4 Figs. The definitive version is available at: https://www.sciencedirect.com/science/journal/03418162, A key problem in explaining the mountain landscapes of the Mediterranean region is the relationship between the development of transhumance (seasonal movement of livestock over long distances) and deforestation of the subalpine and upper montane belts at 1350–2200 m above sea level (m a.s.l.). We examined this relationship in the Central Southern Pyrenees using information from multiple proxies—archaeology, pollen analysis, lacustrine sedimentation patterns, historical documents, and geomorphology. Although there is evidence of forest clearing since the Neolithic, and we can infer the presence of shepherds and livestock in mountain areas based on archaeological sites and pollen analysis in some lacustrine records, there is no strong evidence for transhumance between the Ebro Depression and the Pyrenees during the Prehistory. Instead, we found evidence of transterminance (seasonal movement of livestock over short distances) during this time. Growth of human populations and the establishment of large-scale markets favored longer livestock movements during the Roman period, although we only have indirect information on this for other areas of the Iberian Peninsula and the Mediterranean region. A few documents indicate the occurrence of transhumance during the Early Middle Ages, whereas all other proxies indicate a general expansion of livestock and transhumance during the 12th and 13th centuries. This coincides with the Christian conquest of the Central Ebro Depression by the Aragon Kingdom, the organization of large wool markets in Western Europe, and the exemptions conferred by the kings of Aragon to herders in the city of Saragossa. This led to rapid deforestation, in that forests were rapidly logged or burned to expand the summer grasslands. During this period, written documents indicate many conflicts between people in the villages and valleys for control of summer grasslands, pollen analyses show a rapid decline of arboreal pollen, and geomorphology studies indicate greatly increased sedimentation rates following deforestation, suggesting intense erosion. Transhumance has declined since the end of the 18th century, and particularly throughout the 20th century, and this has led to tree re-colonization of less accessible and marginal areas. The hydrological and geomorphological consequences of these recent events are important topics for future studies., This research was supported by the ESPAS Project (CGL2015-65569-R) and the Gaps and Sites Project (HAR2017-85023-P), both funded by the MINECO-FEDER, and by the Research Group from the Aragon Government (H14_17R). R. Domingo is a Ramón y Cajal Research Fellow (RyC2013-12613, MINECO). P. Diarte-Blasco is an Atracción de Talento-CAM Research Fellow (2018-T1/HUM-10636).

This chapter reviews the most significant aspects of erosion in badlands, one of the most impressive erosion landforms on Earth. Badlands occur in arid, semiarid and humid areas, are characterized by intense erosion processes and generally have extremely high sediment yields. They lead to sedimentation in pediments, high sediment load in fluvial channels and sediment silting in reservoirs. The present study examines the factors that most affect erosion in badlands: lithology, seasonal contrasts in climate, rainfall intensity, and low plant cover, weathering and erosion processes, spatial and temporal variability of erosion processes and the complex spatial and temporal connections at the catchment scale. Two sections examine critical future issues: the relationship between conservation and reclamation in badlands and the future evolution of badland erosion in the context of global change. In many cases, badland initiation and development occurred due to interactions between climate changes and human activities, particularly the use of fire and cultivation on steep slopes of soft bedrock material. The largest badlands should be preserved as geomorphological monuments because of their importance to education, research and tourism.

41 Pags.- 14 Figs.- 1 Tabl. The definitive version is available at: http://www.sciencedirect.com/science/journal/0169555X, In this study the spatial distribution of shallow landslides in the upper montane and subalpine belts of the Urbión Mountains (Iberian Range, northern Spain) was investigated, particularly in relation to the spatial organization of deforestation and land cover. The upper montane and subalpine belts have been deforested several times since the Neolithic Period, to enlarge the area of summer grasslands for feeding transhumant sheep flocks. Consequently, the timberline was lowered by 400–600 m, and increased the occurrence of severe erosion processes, particularly shallow landslides. This study shows that most of the landslide scars are in the summer grasslands area, and that a remarkable extent of the subalpine belt area has been subjected to mass movements. In addition to land use, the soil characteristics and topography help explain the development of conditions most favorable to landsliding. Shallow landslide susceptibility was highest in the upper parts of the slopes near the divides, in areas having slope gradients of 10–30° and deep soils with an increasing proportion of clay with depth. The landslides were clustered and not randomly distributed, and the causes of this spatial distribution are discussed. The current trend of woody encroachment in the upper montane and subalpine belts, resulting from decreasing livestock pressure, will probably reduce the susceptibility of these areas to shallow landslides in the future., Support for this research was provided by the project CGL2015-65569-R (MINECO/FEDER).

Deglaciation processes in the upper Gállego Valley, central-southern Pyrenees, were studied using geomorphological mapping and 36Cl cosmogenic dating of moraine and rock glacier boulders, as well as polished bedrock. Although the precise position of the Gállego Glacier during the global last glacial maximum is not known, there is evidence that ice tongues retreated to the headwaters, which caused subdivision of the main glacier into a number of individual glaciers prior to 17ka. A range of ages (16 to 11ka) was found among three tributary valleys within the general trend of deglaciation. The retreat rate to cirque was estimated to be relatively rapid (approximately 5km per ka). The mapped glacial sedimentology and geomorphology appears to support the occurrence of multiple minor advances and retreats, or periods of stasis during the late deglaciation. Geomorphological and geological differences among the tributary valleys, and error estimates associated with the results obtained, prevented unambiguous correlations of the advances with the late Pleistocene cold periods. During the latter advances, small glaciers and rock glaciers developed close to the cirque headwalls, and co-occurred under the same climatic conditions. No evidence for Holocene re-advance was found for any of the three tributary valleys. © 2015 University of Washington., This study was performed with the support of the projects CGL2006-11619/HID, CGL2011-27753-C02-01 and CGL2012-35858, funded by the Spanish Ministry of Economy and Competitiveness. The research group on Geomorphology and Global Change was financed by the Aragón Government and the European Social Found (ESF-FSE) (E68). The authors acknowledge Dr David Fink, Dr Magali Delmas and an anonymous reviewer for their valuable comments on the manuscript. We also particularly thank Dr Irene Schimmelpfennig, whose collaboration was essential to the study.

Streamflows in a Mediterranean mountain basin in the central Spanish Pyrenees were projected under various climate and land use change scenarios. Streamflow series projected for 2021-2050 were used to simulate the management of the Yesa reservoir, which is critical to the downstream supply of irrigation and domestic water. Streamflows were simulated using the Regional Hydro-Ecologic Simulation System (RHESSys). The results show that increased forest cover in the basin could decrease annual streamflow by 16%, mainly in early spring, summer and autumn. Regional climate models (RCMs) project a trend of warming and drying in the basin for the period 2021-2050, which will cause a 13.8% decrease in annual streamflow, mainly in late spring and summer. The combined effects of forest regeneration and climate change are expected to reduce annual streamflows by 29.6%, with marked decreases affecting all months with the exception of January and February, when the decline will be moderate. Under these streamflow reduction scenarios it is expected that it will be difficult for the Yesa reservoir to meet the current water demand, based on its current storage capacity (476hm3). If the current project to enlarge the reservoir to a capacity of 1059hm3 is completed, the potential to apply multi-annual streamflow management, which will increase the feasibility of maintaining the current water supply. However, under future climate and land cover scenarios, reservoir storage will rarely exceed half of the expected capacity, and the river flows downstream of the reservoir is projected to be dramatically reduced. © 2013 Elsevier B.V., We thank the Spanish Meteorological State Agency (AEMET) for providing the database used in this study. This work was supported by the research projects CGL2011-27536, CGL2011-27753-C02-01 and CGL2011-27574-CO2-02, financed by the Spanish Commission of Science and Technology and FEDER; ACQWA (FP7-ENV-2008-1-212250), financed by the VII framework program of the EC; “Efecto de los escenarios de cambio climático sobre la hidrología superficial y la gestión de embalses del Pirineo Aragonés”, financed by “Obra Social La Caixa”; and CTTP1/12 “Creación de un modelo de alta resolución espacial para cuantificar la esquiabilidad y la afluencia turística en el Pirineo bajo distintos escenarios de cambio climático”, financed by the Comunidad de Trabajo de los Pirineos.

[EN] The erosion and transport of fine-grained sediment in small mountainous catchments involve complex processes occurring at different scales. The suspended sediment yields (SSYs) delivered downstream are difficult to accurately measure and estimate because they result from the coupling of all these processes. Using high frequency discharge and suspended sediment data collected in eight small mountainous catchments (0.45-22km 2) from four distinct regions, we studied the relationships between event-based SSY and a set of other variables. In almost all the catchments, the event peak discharge (Q max) proved to be the best descriptor of SSY, and the relations were approximated by single power laws of the form SSY=αQmaxβ. The β exponents ranged between 0.9 and 1.9 across the catchments, while variability in α was much higher, with coefficients ranging between 25 and 5039. The broad distribution of α was explained by a combination of site-specific physical factors, such as the percentage of degraded areas and hillslope gradient. Further analysis of the factors responsible for data dispersion in each catchment was carried out. Seasonality had a significant influence on variability; but overall, most of the scattering in the SSY-Q max regressions was explained by the short-lasting memory effects occurring between successive events (i.e. in-channel temporary storage and remobilization of sediment; antecedent moisture conditions). The predictability of SSY-Q max models was also assessed. Simulations of SSY per event and of annual SSY were conducted by using the computed regressions and the measured Q max. Estimates of SSY per event were very uncertain. In contrast, annual SSY estimates based on the site-specific models were reasonably accurate in all the catchments, with interquartile ranges remaining in the ±50% error interval. The prediction quality of SSY-Q max relations was partly attributed to the statistical compensation that likely occurred between extreme values over a year; but it also suggests that the complex processes occurring at the event scale were smoothed at the annual scale. This SSY-Q max rating appears as a parsimonious predicting tool for roughly estimating SSY in small mountainous catchments. However, in its current form the technique needs further improvement as α and β values need to be better constrained. © 2012 Elsevier B.V., This research was funded by the following projects: STREAMS (French National Research Agency – BLAN06-1_139157), DESIRE (European Union FP6 – Contract No. 037046), PROBASE and INDICA (Spanish Government Grants No. CGL2006-11619/HID and CGL2011-27753-C02-01, respectively), and RESEL (Spanish Ministry of Environment). The authors wish to thank the CIEco and CIGA (Universidad Nacional Autónoma de México) for a fruitful collaboration. We are grateful to C. Prat for locally coordinating the DESIRE Project and to C. Obled for sharing constructive discussions. D. Tropeano and B. Fahey are also acknowledged for kindly providing documents. E. Nadal-Romero was the recipient of a research contract from the Spanish Ministry of Science and Innovation (Juan de la Cierva Program). Lastly, we would like to thank the two anonymous reviewers for their helpful comments on an earlier draft of this article

66 Pag., 5 Fig. The definitive version is available at: http://www.sciencedirect.com/science/journal/00128252, Mediterranean areas of both southern Europe and North Africa are subject to dramatic changes that will affect the sustainability, quantity, quality, and management of water resources. Most climate models forecast an increase in temperature and a decrease in precipitation at the end of the 21st century. This will enhance stress on natural forests and shrubs, and will result in more water consumption, evapotranspiration, and probably interception, which will affect the surface water balance and the partitioning of precipitation between evapotranspiration, runoff, and groundwater flow. As a consequence, soil water content will decline, saturation conditions will be increasingly rare and restricted to periods in winter and spring, and snow accumulation and melting will change, especially in the mid-mountain areas. Future land management will be characterized by forest and shrub expansion in most Mediterranean mountain areas, as a consequence of farmland and grazing abandonment, with increasing human pressure localized only in some places (ski resort and urbanized of valley floors). In the lowlands, particularly in the coastal fringe, increasing water demand will occur as a consequence of expansion of irrigated lands, as well as the growth of urban and industrial areas, and tourist resorts. Future scenarios for water resources in the Mediterranean region suggest (1) a progressive decline in the average streamflow (already observed in many rivers since the 1980s), including a decline in the frequency and magnitude of the most frequent floods due to the expansion of forests; (2) changes in important river regime characteristics, including an earlier decline in high flows from snowmelt in spring, an intensification of low flows in summer, and more irregular discharges in winter; (3) changes in reservoir inputs and management, including lower available discharges from dams to meet the water demand from irrigated and urban areas. Most reservoirs in mountain areas will be subject to increasing water resource uncertainty, because of the reduced influence of snow accumulation and snowmelt processes. Besides, reservoir capacity is naturally reduced due to increasing sedimentation and, in some cases, is also decreased to improve the safety control of floods, leading to a reduction in efficiency for agriculture. And (4) hydrological and population changes in coastal areas, particularly in the delta zones, affected by water depletion, groundwater reduction and saline water intrusion. These scenarios enhance the necessity of improving water management, water prizing and water recycling policies, in order to ensure water supply and to reduce tensions among regions and countries., This work was supported by research projects CGL2006-11619/HID, CGL2008-01189/BTE, and CGL2008-1083/CLI, financed by the Spanish Commission of Science and Technology, and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1-212250, financed by the European Commission, the VII Framework Programme financed by the European Commission, the project “Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica” and “La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climáticas”, financed by “Obra Social La Caixa” and the Aragón Government, and “Programa de grupos de investigación de excelencia”, financed by the Aragón Government.

11 páginas, 10 figuras, 3 tablas., Soil erosion is a key factor in Mediterranean environments, and is not only closely related to geoecological factors (lithology, topography, and climatology) but also to land-use and plant cover changes. The long history of human activity in Spain explains the development of erosion landscapes and sedimentary structures (recent alluvial plains, alluvial fans, deltas and flat valleys infilled of sediment). For example, the expansion of cereal agriculture and transhumant livestock between the 16th and 19th centuries resulted in episodes of extensive soil erosion. During the 20th century farmland abandonment prevailed in mountain areas, resulting in a reduction of soil erosion due to vegetation recolonization whereas sheet-wash erosion, piping and gullying affected abandoned fields in semi-arid environments. The EU Agrarian Policy and the strengthening of national and international markets encouraged the expansion of almond and olive orchards into marginal lands, including steep, stony hill slopes. Vineyards also expanded to steep slopes, sometimes on new unstable bench terraces, thus leading to increased soil erosion particularly during intense rainstorms. The expansion of irrigated areas, partially on salty and poorly structured soils, resulted in piping development and salinization of effluents and the fluvial network. The trend towards larger fields and farms in both dry farming and irrigated systems has resulted in a relaxation of soil conservation practices., Support for this research was provided by the projects PROBASE (CGL2006-11619/HID, Consolider), financed by the Spanish Commission of Science and Technology, PI032/08 financed by the Aragón Regional Government, and ACQWA (FP7-ENV-2007-1-212250) financed by the European Commission. The author also acknowledges support from the Program of Research Groups of the Aragón Regional Government, and from RESEL (The Spanish Ministry of Environment).

64 Pag., 8 Tabl., 10 Fig., Soil erosion and sediment delivery to streams are important environmental problems and major concerns in sustainable development. In recent years several tools have been proposed for assessing the delivery of sediment from hillslopes to stream networks, but there are still few examples of their application to large basins, and studies include a discussion of calibration and validation issues. In this study a spatially distributed soil erosion and sediment delivery model (WATEM/SEDEM) was applied to the watershed of the Barasona Reservoir (1504 km2, Central Spanish Pyrenees), which is drained by the Ésera and Isábena Rivers. Model calibration and validation was based on the depositional history of the Barasona Reservoir and suspended sediment records over 3 years (May 2005–May 2008) at the outlet of the Isábena River. Despite interannual differences in precipitation and runoff, it was possible to establish an optimum value (7–23) for the ktc parameter. This enabled estimation of the long-term average annual sediment yield to the reservoir, as well as the relative contribution of each river. Large spatial variability in hillslope sediment delivery was found, and the major sediment sources were in the lower part of the watershed (agricultural fields) and in the badlands developed on Eocene marls in the middle part of the watershed (Pyrenean Inner Ranges). The relative importance of sediment source areas was assessed in relation to land use, and the relationship between the sediment delivery ratio and the catchment area was studied as a function of the geomorphological units. For the moment WATEM/SEDEM remains mainly as a researcher’s tool, until either the problem of the scarcity of soil erosion data or the need for calibration of the transport parameters will be solved., This research was financially supported by the projects CGL2006-11619/HID and CGL2008-00831/BTE, funded by CICYT, Spanish Ministry of Education and Science. The contribution of the first author was possible through a scholarship granted by The National Council for Science and Technology of Mexico (CONACYT).

Three small catchments (, Support for this research was provided by the following projects: CANOA (CGL2004-04919-C02-01), PROBASE (CGL2006-11619/HID, Consolider), financed by the Spanish Commission of Science and Technology, Ministry of Education and Science, and the project PM088/2006 financed by CONAI (Aragón Government). The authors also acknowledge support from the Program of Research Groups of the Aragón Regional Government, and from RESEL (the Spanish Ministry of the Environment). J. Latron has benefited from a research contract (Juan de la Cierva Programme), funded by the Spanish Ministry of Education and Science.

1 página.-- Introducción completa en resumen., The papers in this special issue of Catena are a selection of those presented at the Sections 6 and 7 of the 6th International Conference on Geomorphology, held in Zaragoza (Spain) from September 6 to 11, 2005. This Conference joined almost 1000 geomorphologists, many of them working on “border” topics, and therefore contributing to the advance of Geomorphology as a key science to understand the complex evolution and dynamics of landforms, and the interactions between past and present climate changes, land use changes, lithology, geological structure and tectonics. The papers presented at the Conference gave an excellent overview about the contribution of Geomorphology in different fields such as, among others, natural hazards, soil erosion, sediment transport, mass movements, extreme events, Global Change and environmental stress, and their relationships with land management. Sections 6 and 7 were devoted to Hillslope Processes and Soil erosion, respectively. Both sessions were among the most attended of the Conference in number of papers presented and participants, confirming an increasing interest in the study of land degradation, sediment transport and mass movements at different spatial and temporal scales. New experimental, field and laboratory methods, detailed field observations and measurements, and the use of models to explain and predict geomorphic processes were common to many of the papers. Particularly, as previous issues of Catena and other international journals demonstrated, Global Change is present in almost all of the papers dealing on soil erosion and hillslope processes. How would it be possible to explain the characteristics of infiltration, rill, interril and gully erosion, sediment transport and river morphology, or the triggering of different mass movements without any reference to climate and land use changes? Most geomorphic processes are governed by temperature oscillations, the magnitude-frequency of rainfalls and floods, or human activities such as deforestation and reforestation, soil cultivation on steep slopes, farmland abandonment, overgrazing, channelization, gravel mining and construction of dams. An increasing number of papers stressed the role of climate fluctuations and historical and present-day land management, and this is the best contribution the geomorphologists can do in order to not only understand the geomorphic processes, but especially to improve our unique and menaced world. The selection of papers included in this issue of Catena is particularly related to land use/land cover changes. These papers focus on (i) soil erosion processes (desertification, consequences of forest fires, factors controlling soil erosion, evaluation of denudation rates); (ii) gully development (vulnerable areas for gully erosion and gullies under forest canopy); and (iii) catchment studies (sediment transport, sediment delivery, relationships between streamflow and sediment load or simulation of badland erosion at catchment scale). The editors of this special issue wish to acknowledge the efforts made by all authors and particularly the reviewers for their anonymous and invaluable work.

40 Pag., 2 Tabl., 6 Fig. The definitive version is available at: http://www.sciencedirect.com/science/journal/00335894, Palynological, sedimentological and stable isotopic analyses of carbonates and organic matter performed on the El Portalet sequence (1802 m a.s.l., 42°48′00N, 0°23′52W) reflect the paleoclimatic evolution and vegetation history in the central-western Spanish Pyrenees over the last 30,000 yr, and provide a high-resolution record for the late glacial period. Our results confirm previous observations that deglaciation occurred earlier in the Pyrenees than in northern European and Alpine sites and point to a glacial readvance from 22,500 to 18,000 cal yr BP, coinciding with the global last glacial maximum. The patterns shown by the new, high-resolution pollen data from this continental sequence, chronologically constrained by 13 AMS 14C dates, seem to correlate with the rapid climate changes recorded in Greenland ice cores during the last glacial–interglacial transition. Abrupt events observed in northern latitudes (Heinrich events 3 to 1, Oldest and Older Dryas stades, Intra-Allerød Cold Period, and 8200 cal yr BP event) were also identified for the first time in a lacustrine sequence from the central-western Pyrenees as cold and arid periods. The coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the North Atlantic to mid latitudes., This study was partially funded by projects REN 2000-1136/CLI, REN2003-02499-GLO and REN-2003-09130-CO202, by the CICYT and PI-17/00739/FS/01 (Fundación Séneca). Funding for P.G.S. was provided by the Spanish Government (post-doctoral grant in UPS-Toulouse, France).

17 páginas, 14 figuras, 1 tabla.-- Quaternary of the Mediterranean basin and western Asia: marine and terrestrial processes and palaeoenvironments., Though usually under-represented in palaeobiogeographical models, northeastern Spain is one of the regions of southern Europe with the greatest biodiversity. Strong climatic and geographic gradients and topographic contrasts have contributed to the marked physiographic heterogeneity of the region. Pollen sites from the Ebro River Basin (northeastern Spain) have been insufficiently studied, perhaps because of the scarcity of suitable sites for conventional pollen analyses (i.e. lakes, peatbogs) and the characteristic mosaic vegetation patterns. Glacial and Lateglacial pollen records from a variety of archives located along a N–S transect: Glacial lakes, periglacial screes, peatbogs, hyena coprolites and archaeological sites were analyzed. The reconstruction of full and Lateglacial landscapes in the Ebro River valley provides new insights into the vegetational history of the region: (i) the abundance of steppe formations with Artemisia, Poaceae, Chenopodiaceae, Ephedra, Asteraceae during the most intensive cold-arid episodes in both the non-glaciated mountains and the lowlands, (ii) the continuous occurrence of woody taxa even at high areas, (iii) the expansion of these woody taxa during Lateglacial climatic ameliorations, (iv) the survival of mesothermophytes in small refuge areas during full-glacial stages. There was great vegetation diversity in the northeastern Iberian landscapes during the Last Glacial and Lateglacial stages, suggesting patches of vegetation with steppic areas, Mediterranean shrubs, coniferous forest and refuges of mesothermophilous taxa., This study was partially funded by the projects ‘‘Arid periods in the Mediterranean area since the Last Glacial Maximum’’ (REN2000-1 136/CLI) and ‘‘Limnocliber’’ (REN2003-0 9130-CO2-02), funded by the CICYT. We are also grateful to the Pyrenean Institute of Ecology (CSIC) for financial support. J.S. Carrión acknowledges funding by the projects PI-17/00739/FS/01 (Fundación Séneca) and REN2003-02499-GLO (Spanish Office of Science and Technology).

22 Pag., 3 Tabl., 7 Fig., 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 millimeters 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., This paper has been supported by the following projects: “Water resources management in a changing environment: the impact of sediment on sustainability” (WARMICE, ENV4-CT98-0789) funded by the European Commission; and “Assessment of sediment sources and runoff generation areas in relation to land use changes” (HIDROESCALA, REN2000-1709-C04-01/GLO), “Hydrological processes in seminatural Mediterranean areas” (PROHISEM, REN 2001-2268-C02-01/HID), and “Hydrological processes in Pyrenean catchments in relation to land use and climate changes” (REN 2003-08678/HID), all funded by the CICYT. Monitoring of the catchment has been supported by agreement between the CSIC and the Spanish Ministry of Environment (RESEL).