Reconstruction ensembliste des événements spatio-temporels d'étiage extrême en France depuis 1871 | Ensemble reconstruction of spatio-temporal extreme low-flow events in France since 1871

This research presents a study of extreme low flow events that occurred from 1871 onwards in a large number of near-natural catchments in France. It aims at assessing and comparing their characteristics to improve our knowledge on historical events and to provide a selection of benchmark events for climate change adaptation purposes. The historical length of streamflow observations is generally limited to the last 50 years and therefore covers too small a sample of extreme low flow events to properly explore the long-term evolution of their characteristics and associated impacts. In order to overcome this limit, this study takes advantage of a 140-year ensemble hydrometeorological dataset over France based on: (1) probabilistic precipitation and temperature downscaling of the global Twentieth Century Reanalysis over France (Caillouet et al., 2016a), and (2) continuous hydrological modelling based on high-resolution meteorological reconstructions as forcings over the whole period. The resulting SCOPE Hydro dataset provides an ensemble of 25 equally plausible daily streamflow time series for a reference network of more than 600 stations in France over the 1871-2012 period. Extreme low-flow events are identified using a combination of a fixed threshold and a daily variable threshold. The procedure is applied to the 25 simulated time series as well as to the observed time series in order to compare observed and reconstructed events over the recent period, and to characterize in a probabilistic way unrecorded historical events. A spatial matching procedure at the scale of France is then developed in order to spatially assemble local extreme events into the same spatio-temporal event. After these steps, an event can be studied at the local or national scale through its spatial extent, duration or severity characteristics in a probabilistic setting (Caillouet et al., 2016b). This work identifies past and little known exceptional extreme events (1878, 1893, 1942-1949) or recent but poorly documented events (1972, 1978, 1985) besides well-known events (1921, 1976, 1989-1990, 2011). The evolution of these events since 1871 shows that an increasing part of the French territory is affected by extreme low-flow events since the 1940s. Finally, a good coherence is found between reconstructed events and narrative sources on historical droughts. For the first time, extreme low-flow events are described in a homogeneous manner over 140 years for a large set of near-natural French catchments, enabling detailed analyses of the effect of climate variability and anthropogenic climate change on low-flow hydrology.