Adrián Lorente

Assessing the Effect of Climate Oscillations and Land-use Changes on Streamflow in the Central Spanish Pyrenees

Abstract Plans to increase the amount of irrigated land in Mediterranean countries should consider how changes in climate and land-use affect water resources. In this study, both precipitation and temperature were used to analyze regional trends in discharge in the basins of the Central Spanish Pyrenees since the mid-20th century. Annual variations in the relationship between precipitation and discharge suggested that discharge was relatively lower in the second half of the study period, coinciding with major changes in land use. On a monthly scale, precipitation increased significantly in October, April, and July, and decreased in March, and temperature increased in January and February and decreased in April. Nevertheless, discharge has decreased significantly in most months in the past 50 years. Land-use and plant-cover changes are the only nonclimatic factor that can explain the loss of around 30% of the average annual discharge.

Uncertainty assessment in the prediction of extreme rainfall events: an example from the central Spanish Pyrenees

Extreme rainfall events occur frequently in the central Pyrenees, but they are responsible for mass movements and short, very intense erosion periods, accompanied at times by loss of human life and high costs of infrastructure. This paper tries to assess the existence of patterns in the spatial distribution of maximum precipitation. The calculation of return periods of the most intense rainfall demonstrates that in the Pyrenees it exhibits an erratic spatial and temporal distribution and can be extremely localized. In the case of precipitation between 150 and 200 mm in 24 h, some influence from the surrounding relief has been found, but this is not the case for precipitation exceeding 200 mm, characterized by the absence of patterns governing their spatial distribution. Geomorphological approaches are, therefore, the only way for assessing the areas more subject to hydromorphological risks. Copyright

Factors Explaining the Spatial Distribution of Hillslope Debris Flows

Abstract The spatial distribution of 961 debris flows in the Upper Aragón and Gállego valleys (Central Spanish Pyrenees) was analyzed. Most were located in the Flysch Sector (with a colluvium mantle derived from strongly tectonically modified materials), between 1000 and 1400 m above sea level, on 25–35° gradients with sunny exposure. These gradients were either hillslopes covered by frequently burned scrubland, abandoned fields, or reforested land, confirming the influence of land use and disturbed landscapes on the occurrence of debris flows.

Geomorphological consequences of fraquent and infrequent rainfall and hydrological events in Pyrennez Mountains of Spain

Various aspects of pluviometric andhydrological events have been studiedworldwide, one of which is the geomorphichazards as the intensity of the eventsexceeds various geomorphic thresholds.During the last few years, rainstorms ofdifferent intensities have occurred in theCentral Spanish Pyrenees, including one ofexceptional character. Large, historicaldebris flows have been studied, as well asthe actual sediment transport in smallexperimental catchments. This study showsthat during the most frequent eventssuspended sediment transport is the commongeomorphic process. Bedload is mobilizedseveral times per year while small rockavalanches and channelized debris flowshave a return period of at least 5 years.Hillslope debris flows are triggered byrainfall events with a 25–30 year returnperiod. Reactivation of large, deep massmovements is linked to rainfalls of around100 year return period (between 130 and160 mm in 24 hours). Catastrophicgeomorphic processes occur whenprecipitation exceeds a 100 year returnperiod, as was the case of the Biescascampsite disaster. Geomorphic processestriggered by intense rainfall events havecaused major damage and human disastersbut the hazards have been reduced by theintroduction of several control measures,including reforestation, the constructionof check-dams, canalization of riversegments and improved flood forecasting.

Factors in peak flows generation in an experimental catchment of the Central Pyrenees, Spain

Se han estudiado los factores que explican la variabilidad temporal de los picos de crecida en una cuenca experimental del Pirineo Central. Esta cuenca (Arnas, Valle de Borau) fue completamente cultivada hasta hace unos 50 anos y se caracteriza por una respuesta hidrologica casi inmediata frente a cualquier intensidad y volumen de precipitacion. Se ha comprobado que la cantidad total de precipitacion durante cada evento apenas explica la intensidad de los picos de crecida, que muestran una variabilidad muy alta. Las relaciones entre precipitacion y caudal mejoran sensiblemente si se separan las crecidas de estacion fria y de estacion calida. La lluvia registrada durante 3 y 7 dias antes de cada crecida tiene un papel relativamente importante en invierno. Durante el verano y principio de otono la intensidad de los picos de crecida no se relaciona con ninguna variable pluviometrica debido a la fuerte evapotranspiracion y a la separacion existente entre eventos lluviosos