Climate change impact on the discharge of the Italian river network using Convection Permitting climate models

Projecting the impact of climate change on the Mediterranean hydrological cycle and its extremes can be challenging, notably in regions of complex topography like the Italian peninsula. For the medium (≤71.000km2) to small basins (≤30km2), kilometer scale resolution climate projections are needed to obtain high resolution spatio-temporal information and capture possible changes in local hydrology. A regional climate model ensemble for the Great Alpine region was made available, at the convection permitting (CP-RCM) resolution (<3 km), by the CORDEX Flagship Pilot Study on Convective phenomena at high-resolution over Europe and the Mediterranean (FPSCONV). The CETEMPS hydrological model CHyM has been used coupled off-line with the different CP-RCM ensemble members under two different configurations: using temperature and precipitation from the driving CP-RCM or directly its runoff. The results project a 10-25% decrease in yearly mean discharge by the end of the century, with a 25-50% decrease in summer mean discharge in most of Italy. Extreme events also increase, with 25% more discharge for the 100-yr return period and 25% less discharge in the 10-yr return period discharge. Compared with hydrological ensembles at lower resolution, the main differences are found over complex topography, where, for example, the CP-RCM ensemble shows a lengthening of the hydrological year and more intense extremes. This study highlights the importance of high-res climate projections to assess future hydrological changes in Mediterranean regions, showing reductions in mean river discharge, intensification of extreme events and change of the characteristic of the hydrological year by 2100.