Climate change impact on flood hazard over Italy

This study assesses future projections of flood hazards across the Italian river basins. For this purpose, sub-daily river discharge for the entire Italian territory was simulated with the CETEMPS hydrological model (CHyM) using as climate forcing both observational datasets and regional climate simulations completed with the ICTP Regional Climate Model (RegCM4). First, simulated precipitation and river discharge were evaluated against observational products, showing both models good performance in simulating Italian precipitation and discharge characteristics. Then, projections of these variables were explored for two time slices, near (2020-2049) and far (2070-2099) future, under the RCP8.5 emission scenario, with RegCM4 driven by output from a global climate model projection. The RegCM4 outputs were also bias-adjusted by means of the N-dimension multivariate bias correction (MBCn) method. We found that, although the bias correction leads to a benefit in capturing mean precipitation, the improvement is less clear for extreme precipitation. In terms of discharge, however, bias correction has a noticeable impact on mean discharge as well as average yearly peak flow and hourly peak flow at return periods of 10, 50, and 100 years. Nevertheless, for a large part of the Italian territory, projected changes in mean precipitation and discharge do not show significant differences when bias correction is applied, while for extreme events only similar to 30 % of river points show significant sensitivity to bias correction. Under the RCP8.5 scenario, an increase in yearly peak flow is evident over most of the Italian peninsula, with values increasing from 30 % to more than 100 % for the mid and far time slices respectively, especially over the Po River and along the eastern coasts of Italy. Similar conclusions are found for the hourly peak flows of 10, 50, and 100-year return periods. These indicators thus suggest a general increase in flood hazard expected over the region.