A 50-year analysis of hydrological trends and processes in a Mediterranean catchment

The Réal Collobrier hydrological observatory in south-eastern France, managed by Irstea since 1966, constitutes a benchmark site for regional hydro-climatology. Because of the dense network of stream gauges and rain gauges available, this site provides a unique opportunity to evaluate long-term hydro-meteorological Mediterranean trends. The main catchment (70 km²) and its sub-catchments are located in the massif des Maures of south-eastern France, close to the Mediterranean coast. The vegetation is composed of forest mainly calcified on crystalline soils (maquis of heath, cork-oak, maritime pine and chestnut). Direct human influence has been negligible over the past 50 years. The land use and land cover has remained almost unchanged, with the notable exception of a wildfire in 1990 that impacted a small sub-catchment. Therefore changes in the hydrological response of the catchments are caused by changes in climate and/or physical conditions. This study investigates changes in observational data using up to 50-year daily series of precipitation and streamflow. The analysis used several climate indices describing distinct modes of variability, at inter-annual and seasonal timescales. Trends were assessed by the Mann-Kendall method. The analysis also used hydrological indices describing drought events based on daily data for a description of low flows, in particular in terms of timing and severity. The analysis shows that there is a marked tendency towards a decrease in the water resources of the Réal Collobrier catchment in response to climate trends, with a consistent increase in drought severity and duration. But the changes are variable among the sub-catchments. L'observatoire hydrologique du Réal Collobrier dans le sud-est de la France, géré par Irstea depuis 1966, constitue un site de référence en hydro-climatologie régionale. En raison du réseau dense de stations de jaugeages du cours d'eau et de pluviomètres disponibles, ce site offre une occasion unique d'évaluer les tendances hydrométéorologiques méditerranéennes à long terme. Le bassin versant principal (70 km²) et ses sous-bassins versants sont situés dans le massif des Maures du sud-est de la France, à proximité de la côte méditerranéenne. La végétation est composée de forêts principalement calcifiées sur des sols cristallins (maquis de bruyère, chêne-liège, pin maritime et châtaignier). L'influence humaine directe a été négligeable au cours des 50 dernières années. L'utilisation et la couverture