The length of streamflow observations is generally limited to the last 50 years even in data-rich countries like France. It therefore offers too small a sample of extreme low-flow events to properly explore the long-term evolution of their characteristics and associated impacts. To overcome this limit, this work makes use of a daily 140-year ensemble streamflow reanalysis for a reference network of 661 near-natural catchments in France. This reanalysis, called FYRE Hydro, is based on (1) the high-resolution surface reanalysis FYRE Climate combining the outputs of a probabilistic downscaling of the Twentieth Century Reanalysis and historical observations from the Meteo-France database, (2) a continuous hydrological modelling using this reanalysis as forcings over the whole period and (3) a data assimilation scheme using all available historical streamflow observations. This work makes use of tools defining spatio-temporal extreme low-flow events. Extreme low-flow events are first locally defined through the Sequent Peak Algorithm using a novel combination of a fixed threshold and a daily variable threshold. A dedicated spatial matching procedure is then established to identify spatio-temporal events across France. This procedure is furthermore adapted to the ensemble dataset to characterise in a probabilistic way unrecorded historical events at the national scale. Extreme low-flow events are described and compared in a spatially and temporally homogeneous way over more than 140 years on the whole set of 661 catchments. Many different results can be derived from this work, allowing to better understand and characterise extreme low-flow events. Local severity, duration, dynamics or spatial extent are obtained for each station. The spatial characterisation provides a national outlook of such extreme events, with the region of incidence, the percentage of the territory affected, or the temporal evolution. Applied to the last 140 years, results highlight well-known events like 1949 or 1989-1990, but also older and relatively forgotten events like the 1878 and 1893 events. These results contribute to improving our knowledge of historical events and provide a selection of benchmark events for climate change adaptation purposes.
Caillouet, L; Vidal, JP; Sauquet, E; Devers, A; Lauvernet, C; Graff, B; Vannier, O
Inter-comparison of extreme low-flow events in France since 1871
Lhb-Hydroscience Journal