Communicating uncertainties in flood early warnings

A key problem in flood early warnings for small catchments is the increasing forecast uncertainty associated with decreasing catchment size. Forecasts for small catchments often are based on rainfall-runoff models and quantitative precipitation estimation, both of which are subject to considerable uncertainties. In the case of a flood event, these uncertainties must be communicated to the responsible authorities along with the actual flood alert, in order for them to respond appropriately.This requires a reliable warning system that can easily adapt to the different needs of its users. Within the HoWa-PRO project (howa-pro.sachsen.de) an information platform (howapro.de) was developed meeting exactly these requirements. The platform is an interactive web-based application providing flood early warnings for small catchments (for now only in Saxony) along with forecast uncertainties. It uses a hydrological ensemble forecast system to calculate hourly predictions and presents these as coloured symbols and plots with uncertainty bands or data layers with various information on probabilities and daily sums.The users of the HoWa information platform are flood risk managers at municipal and district level, who often have no scientific background and certainly, no knowledge of the uncertainties in hydrological forecasts. For this reason, we developed a modular training concept that takes into account the heterogeneous level of knowledge of various user groups, such as employees of water authorities or members of the water brigade and enables a flexible and user-adapted implementation of the training courses. Additionally, a serious game on flood forecasting was designed aiming to convey important key-massages in a playful way. The game can be played during the trainings or just for fun when browsing through the HoWa information platform.Both, the information platform and the training concept were developed in collaboration with the target groups through workshops with intensive discussions and iterative improvements. Consequently, the products on the HoWa platform are visualized and explained in a user-oriented manner. Users' preferences and suggestions were considered in the presentation of warnings (What is presented? How is it presented?) and in communicating uncertainties (Which formulations are also understood by laypersons?). Subsequent to the user workshops, the training concept and information platform were gradually adapted.This contribution provides an overview of the visualization of uncertainties on the HoWa information platform, the training modules on the topic of uncertainties, and the iterative adaptation process by means of a continuous user dialogue.