La prévision hydro-météorologique à 3-6 mois. Etat des connaissances et applications. (Seasonal forecasting of river-flows - State-of-the-art and applications)

This paper focuses on the methodology and tools used in seasonal forecasting of river flows. After a brief state of the art, two applications for predicting low flows of French rivers are presented. The first one aims at forecasting the probability of occurrence of severe droughts using weather typing within hydrologically homogeneous regions. The relatively poor quality of results in validation suggests that France may not be well suited for this type of approach. The second approach is applied to the Rhine River. The rainfall-runoff model GR4J combined with the HBV snow module is forced by a stochastic rainfall generator to provide daily river flows on seven sub-basins for the following 90 days. Results show that a reliable mid term forecasting (< 30 days) is possible in the lower catchment but the tool is not able to anticipate low flows in the upper catchment.