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Abstract:

The Mediterranean region of France experiences flash flooding events when heavy autumn rains occur over small, steep watersheds. The evolution of these events under the influence of climate change is an important question for this region where heavy population growth increases the risks to human lives. The objective of this thesis is to develop a methodology adapted to the study of climate change impacts on small catchments in southern France. The methods developed here must be appropriate for the study of climate change at the spatial and temporal scales of small Mediterranean catchments. A case study was performed on the Lez catchment, near the city of Montpellier in southern France. Precipitation fields input to a hydrological model of the Lez catchment were simulated by the ALADIN regional climate model at a 12 km resolution. Future precipitation fields were bias-corrected using a quantile perturbation-based method. The hydrological model simulated the impact of climate change using bias–corrected precipitation fields.

The hydrological model selected for this study is conceptual, distributed and event–based. The model is based on the SCS equations and uses a series of reservoirs to model the hydrological response of the catchment. This model is sensitive to the hydric state of the catchment, which is correlated with the model initialisation. For the present period, the uncertainty associated with radar precipitation fields input to the hydrological model was reduced by assimilating discharge observations at the catchment outlet. This study also allowed for a better understanding of the level of uncertainty associated with radar rainfall. In light of these results, ground rainfall, which has a lower spatial and temporal resolution, but contains less error, was selected as the reference for the study of climate change impacts on flash floods.

A study of the added value of high resolution regional climate modelling for the reproduction of extreme precipitation was completed. The goal of this study was to estimate the capacity of ALADIN 12 km to simulate extreme precipitation associated with flash flooding in southern France. The 12 km simulations were compared to results from the same model at a 50 km resolution. An atmospheric analysis provided boundary conditions over the period from 1979–2013. ALADIN 12 km improved the distribution of precipitation over France and the simulation of upper precipitation quantiles in southern France. The sources of added value in this study were most likely the improved representation of the topography and changes to the sub-grid precipitation.

The climate change impact study used the RCP4.5 and RCP8.5 scenarios simulated by ALADIN 12 km. Precipitation fields were bias–corrected using a quantile perturbation-based method. This correction transfers the signal of climate change to observed precipitation episodes associated with flood events during the reference period. Using this method, key flood events from the past can be modelled under the influence of climate change. Early results suggest that heavy rainfall and flooding may increase in the Lez catchment in the future. However these results are extremely sensitive to the hydric state of the catchment, assumed stationary in this study. Furthermore, this study used only one member of one global climate model as input to the regional climate model, which is not sufficient to estimate the impact of climate change. Nonetheless, the results of this study show that it is possible to simulate the impacts of climate change in a small, flashy catchment using an event–based methodology.