Towards using subseasonal-to-seasonal ( s 2 s ) extreme rainfall forecasts for extended-range flood prediction

The Arzal dam is located at the mouth of the Vilaine river basin, in Brittany, France, just before the Atlantic Ocean. It creates a 50 Mm3 reservoir of freshwater that supplies the region with drinking water. The dam and its reservoir also play an important role in flood protection, fish migration and touristic activities such as boating. Dam managers have to integrate these multiple uses in their decisions on opening locks and gates. More specifically in summer, they have to guarantee a minimum water volume in the reservoir for water supply, while limiting the entrance of salt water from the ocean when, for instance, allowing boats to leave or enter the reservoir. In periods of low rainfalls and high water demands (May to October), severe low flow situations may occur and uses can become conflictual. A flow forecasting and water management system is essential to provide real-time information to managers and users, supporting decisions and crisis management. The development of this system started in 2013 and comprises two main steps. First, a seasonal forecasting system of inflows upstream the dam was set up and evaluated. Several methods to forecast inflows to the reservoir based on precipitation and streamflow climatology, but also based on seasonal forecasts issued by ECMWF System 4 were analysed. Pre-processing and scenario combination were evaluated for forecast sharpness and reliability. Forecasts of variables of interest to quantify the severity of low flows were incorporated in risk visualisation tools, which were set up with the stakeholders responsible for the management of the dam. In a second step, the management of the reservoir was investigated. Past records of management rules were modelled to reproduce the water levels observed over the summer period. The quality of the seasonal forecasts developed in the previous step was also assessed in this operational context. The main results from the implementation of this low flow forecasting and reservoir water management system will be presented and discussed. The Arzal dam is one of the pilot sites of the Interreg IVB DROP project, which partly funds this research (http://www.dropproject.eu/).