Changes in flood-generating processes in France

The impact of climate change on flooding is quite contrasted in different regions, and often the trends observed can be explained by differentiated changes in flood-generating processes. This study is based on an unprecedented exercise to model the impacts of climate change on hydrology in France, using a semi-distributed model (GRSD) applied to 3727 basins with 22 climate simulations for two greenhouse gas emission scenarios, the RCP4.5 and RCP8.5. Annual maximum flows were extracted for the period 1975-2100, and trend analysis was carried out on both flood magnitude and flood generation processes. Trends in flood magnitude are contrasted, with increasing trends only in the northern half of France, but multi-model convergence rarely exceeds 60%. The increase is greatest for the rarest floods and under the RCP8.5 scenario. For the southern regions, there is an overall decreasing trend in flood magnitudes. Antecedent soil moisture trends follow the same spatial pattern, with an increase mainly in the north-east of France, and decreasing trends in southern basins. The fraction of direct runoff contribution during floods rises sharply in the northwest and the Alps mountains, while the contribution of snowmelt is decreasing in all mountainous regions. Regarding changes in flood-generating processes, the proportion of floods linked to soil saturation excess is increasing mostly in north-eastern France, while decreasing in the south. Conversely, the ratio of floods induced by short and intense rainfall events is increasing in southern and north-western France, most notably under the RCP8.5. The number of rain-on-snow and snowmelt-driven episodes is decreasing across the whole country. This type of approach makes it possible to disentangle the relative influence of different flood-generating processes on trends in flood risk and consequently attribute these changes.