Estimation and communication of uncertainties related to hydrological forecasts for a better information sharing

To prepare the modernization of the flood vigilance system (www.vigicrues.gouv.fr) and more generally to improve the French flood forecasting system, the SCHAPI(3) has launched in 2012 a reflection on the estimation and communication of uncertainties related to hydrological forecasts. Two think tanks were created for this purpose. The first, composed of members of the SCHAPI's scientific board had to answer to three questions, namely: what uncertainty level is acceptable for the end-users, how should uncertainties be estimated and how should they be represented to be communicated. The second think tank, composed of engineers and forecasters from the SCHAPI and flood forecasting services, had to suggest and test technical solutions for the evaluation and the publication of forecasting uncertainties. The main outcomes of these two think tanks are presented hereafter. They give a glimpse of the possible evolutions of flood forecasting products in the near future. It is important to note, that the main obstacles to the communication of forecasting uncertainties are not technical but rather institutional and sociological. The expectations as well as the uses of the forecasts will have to be rethought in depth to account for the new kind of information represented by uncertainties. Therefore, in parallel with the technological developments, an important preparation work, conducted with the end-users, will be necessary before forecasting uncertainties can efficiently be communicated.