Specific Climate Classification for Mediterranean Hydrology and Future Evolution Under Med-CORDEX RCM Scenarios
Authors: Allam Antoine , Moussa Roger , Najem Wajdi , Bocquillon Claude 
Published: March 2020

Abstract. The Mediterranean 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 of 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 hydrological (water resources management, floods, droughts, etc.), and ecohydrological applications such as Mediterranean agriculture like olive cultivation and other environmental practices. 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 classes 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 dry class 1 in the south to the least seasonal and most humid class 5 in the North, showing up 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-km and 50-km resolution simulated under RCP 4.5 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 are increasing seasonality index Is by +80 % and aridity index IArid by +60 % in the North and IArid by +10 % without Is change in the South, hence causing the wet seasons shortening and river regimes modification with the migration North of winter moderate and extreme winter regimes instead of early spring regimes. ALADIN and CCLM RCM models 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 sure 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 same or other climatic indices specific for each region highlighting their physiographic characteristics and hydrological response.