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The significance of predicted climatic changes is still uncertain. The hydrological consequences of climatic changes on Lebanese catchments are analysed by means of different scenarios of rainfall variability and temperature increase. The conceptual rainfall-runoff model MEDOR, coupled to a stochastic model of rainfall and temperature, is used to estimate change in runoff by simulation of six scenarios. These test the response to the rainfall structure, to the duration of rainy events, their frequency, and the duration of the rainy season. The climate-runoff model is used to determine the impact of a temperature increase of 2 degrees on the flow characteristics of a watershed affected by seasonal snow cover. The modifications of the hydrological regimes are significant: droughts are predicted to occur 15 days to one month earlier; snowmelt floods are often replaced by rainfall floods; and the peak flow occurs two months earlier. These changes could have a great impact on water resources management in the future.Key words climate change; simulations; streamflow impact; snow; Mediterranean Simulations des impacts hydrologiques du changement climatique sur les fleuves côtiers LibanaisRésumé La significativité des changements climatiques prévus reste incertaine. Les conséquences hydrologiques des changements climatiques sur des bassins versants Libanais sont analysées au moyen de différents scénarios de variabilité de la pluie et d'augmentation de la température. Le modèle pluiedébit conceptuel MEDOR, couplé à un modèle stochastique de pluie et de température, est utilisé pour estimer le changement dans l'écoulement via la simulation de six scénarios. Ceci permet de tester la réponse à la structure de la pluie, à la durée et la fréquence des événements pluvieux, et à la durée de la saison pluvieuse. Le modèle climat-débit est utilisé pour déterminer l'impact d'une augmentation de la température de 2 degrés sur les caractéristiques de l'écoulement d'un bassin versant présentant un couvert neigeux saisonnier. Les modifications des régimes hydrologiques sont significatives: les étiages sont plus précoces de 15 jours à un mois; les crues nivales sont souvent remplacées par des crues pluviales; et les écoulements extrêmes apparaissent deux mois plus tôt. Ces changements pourraient avoir un grand impact sur la gestion des ressources en eau dans le futur.

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Abstract. The Mediterranean region is one of the most sensitive regions to anthropogenic and climatic changes, mostly affecting its water resources and related practices. With multiple studies raising serious concerns about climate shifts and aridity expansion in the region, this one aims to establish a new high-resolution classification for hydrology purposes based on Mediterranean-specific climate indices. This classification is useful in following up on hydrological (water resource management, floods, droughts, etc.) and ecohydrological applications such as Mediterranean agriculture. Olive cultivation is the characteristic agricultural practice of the Mediterranean region. The proposed approach includes the use of classic climatic indices and the definition of new climatic indices, mainly precipitation seasonality index Is or evapotranspiration threshold SPET, both in line with river flow regimes, a principal component analysis to reduce the number of indices, K-means classification to distribute them into classes, and finally the construction of a decision tree based on the distances to class kernels to reproduce the classification without having to repeat the whole process. The classification was set and validated by WorldClim-2 at 1 km high-resolution gridded data for the 1970–2000 baseline period and 144 stations' data over 30 to 120 years, both at monthly time steps. Climatic classes coincided with a geographical distribution in the Mediterranean ranging from the most seasonal and driest class 1 in the south to the least seasonal and most humid class 5 in the north, showing the climatic continuity from one place to another and enhancing the visibility of change trends. The MED-CORDEX ALADIN and CCLM historical and projected data at 12 and 50 km resolution simulated under the RCP4.5 and 8.5 scenarios for the 2070–2100 period served to assess the climate change impact on this classification by superimposing the projected changes on the baseline grid-based classification. RCP scenarios increase the seasonality index Is by +80 % and the aridity index IArid by +60 % in the north and IArid by +10 % without Is change in the south, hence causing the wet season shortening and river regime modification with the migration north of moderate and extreme winter regimes instead of early spring regimes. The ALADIN and CCLM regional climate models (RCMs) have demonstrated an evolution of the Mediterranean region towards arid climate. The classes located to the north are slowly evolving towards moderate coastal classes, which might affect hydrologic regimes due to shorter humid seasons and earlier snowmelts. These scenarios might look favourable for Mediterranean cultivation; however, the expected impact on water resources and flow regimes will surely expand and directly hit ecosystems, food, health, and tourism, as risk is interconnected between domains. This kind of classification might be reproduced at the global scale, using the same or other climatic indices specific to each region, highlighting their physiographic characteristics and hydrological responses.