Until the middle of the 20th Century, Pyrenean rivers were characterized by braided channels, unstable sedimentary structures and an almost complete lack of plant cover in the alluvial plain, due to the high sediment yield in hillslopes and the occurrence of frequent and intense flooding. This was probably related to strong demographic pressures, including the cultivation of steep slopes, frequent fires, deforestation and overgrazing. Depopulation and farmland abandonment resulted in plant recolonization in formerly cultivated areas, causing a decrease in runoff and sediment yield. As a consequence, most Pyrenean rivers tend to reduce the width of the alluvial plain and to replace the braided pattern with an incised, somewhat meandering pattern, involving the construction of new terrace levels and the stabilization of fluvial bars.

An exceptional rainfall and hydrological event occurred on 19-21 October 2012 in the central western Pyrenees and was particularly significant in the Upper Aragón River basin and its tributaries, mainly the Irati River. Analysis of historical records showed that, considered separately, the event of 19 and 20 October ranked between the second and fifth highest most extreme daily precipitation events. For the two days combined (with a total between 200 and 260 mm), the precipitation event was the most extreme 2-day event among all observation stations but one. The consequent flood destroyed part of an urban area, and a long stretch of a national road triggered landslides, enlarged the alluvial plain and caused generalised soil erosion in cultivated fields cropped with winter cereals. Badlands in the marls of the Inner Depression yielded high volumes of sediment. The floods in the tributaries were relatively moderate (return periods of 14-42 years), whereas in the Upper Aragón River, the flood corresponded to a return period of approximately 400-500 years and to 142 years at the end of the Yesa reservoir, although difficulties in estimating the discharge increased the uncertainty of these values. The Yesa and Itoiz reservoirs considerably reduced the intensity of the flood in the middle and lower Aragón River and confirmed the importance of the water level in the reservoirs when such rainfall events occur. The water storage in the Yesa reservoir increased from 16% to 53% as a consequence of the event. More integrated studies are necessary to decrease the risks associated with flood hazards. This is particularly the case in mountain areas, where the steep slopes and longitudinal gradients of the rivers shorten the concentration time of floods and increase the energy that erodes channels and riverbanks.

Abstract:The temporal and spatial dynamics of groundwater was investigated in a small catchment in the Spanish Pyrenees, which was extensively used for agriculture in the past. Analysis of the water table fluctuations at five locations over a 6-year period demonstrated that the groundwater dynamics had a marked seasonal cycle involving a wetting-up period that commenced with the first autumn rainfall events, a saturation period during winter and spring and a drying-down period from the end of spring until the end of the summer. The length of the saturation period showed great interannual variability, which was mainly influenced by the rainfall and evapotranspiration characteristics. There was marked spatial variability in the water table, especially during the wetting-up period, which could be related to differences in slope and drainage area, geomorphology, soil properties and local topography. Areas contributing to runoff generation were identified within the catchment by field mapping of moisture conditions. Areas contributing to infiltration excess runoff were correlated with former cultivated fields affected by severe sheetwash erosion. Areas contributing to saturation excess runoff were characterized by a marked spatial dynamics associated with catchment wetness conditions. The saturation spatial pattern, which was partially related to the topographic index, was very patchy throughout the catchment, suggesting the influence of other factors associated with past agricultural activities, including changes in local topography and soil properties. The relationship between water table levels and stream flow was weak, especially during the wetting-up period, suggesting little connection between ground water and the hydrological response, at least at some locations. The results suggest that in drier and human-disturbed environments, such as sub-Mediterranean mountains, saturation patterns cannot be represented only by the general topography of the catchment. They also suggest that groundwater storage and runoff is not a succession of steady-state flow conditions, as assumed in most hydrological models.

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Runoff and sediment transport during the snowmelt period (May-June, 2004) in the Izas catchment (Central Pyrenees) were studied to obtain a sediment balance and to assess the relative importance of sediment transport during that period. The results demonstrated the importance of the snowmelt period and showed that most sediment was exported in the form of solutes (76% of the total for the May-June time span); 24% was exported as suspended sediment and no bedload was recorded. Suspended sediment mostly occurred during the second phase of the snowmelt period, when an expanding area of the catchment was free of snow. Sediment transport during the snowmelt period represented 43% of the annual sediment yield. RESUMEN. Se ha estudiado el transporte de sedimento y la escorrentía durante el período de fusión de nieve (mayo-junio de 2004) en la cuenca experimental de Izas (Pirineo Central), con el fin de (i) obtener balance de sedimento y (ii) evaluar la importancia relativa del período de fusión en el transporte total de sedimento a lo largo del año. Los resultados demuestran, por un lado, la importancia hidromorfológica del período de fusión y, por otro, que la mayor parte del sedimento fue exportado en forma de solutos (76% del total durante el período de mayo a junio); el sedimento en suspensión representó el 24%; finalmente, no se registró transporte de carga de fondo. El sedimento en suspensión fue principalmente movilizado durante la segunda parte del período de fusión, cuando una creciente superficie