Will rivers become more intermittent in France? Learning from an extended set of hydrological projections

Abstract. This study aims to assess the changes in the intermittence of river flows across France in the context of climate change. Projections of flow intermittence are derived from the results of the Explore2 project, which is the latest national study that proposes a wide range of potential hydrological futures for the 21st century. The multi-model approach developed within the Explore2 project enables uncertainties in future flow intermittence to be characterized. Combined with discrete observations of flow states, hydrological projections are post-processed to compute the daily probability of flow intermittence (PFI) on each element of the partition of France in hydro-ecoregions (HERs). The post-processing consists of calibrating logistic regressions between the historical flow states of the National Low-Flow Observatory (ONDE) network and the flow data simulated by the hydrological models involved in Explore2 run with the SAFRAN atmospheric reanalysis as inputs. After calibration, these regressions are used to project daily PFIs for the entire 21st century, based on flow simulations from five hydrological models driven by up to 17 climate projections under RCP2.6, RCP4.5, and RCP8.5 climate change scenarios. The results show good agreement among the hydrological models regarding the increase in flow intermittence under RCP4.5 and RCP8.5. The projected increase in mean daily PFI between July and October and the shift of the first and last days when PFI exceeds 20 % both suggest a gradual intensification and extension of dry spells throughout the century. The southern regions of France are likely to experience greater increases in runoff intermittence than the northern regions, and mountainous regions such as the Alps and the Pyrenees are likely to experience changes in their dynamics of intermittence with a reduction in winter intermittence and the apparition of or increase in summer intermittence. The uncertainty of these projected changes is larger in northern France due to greater intermodel variability in this region.