Comparing Flood Forecasting and Early Warning Systems in Transboundary River Basins

This study compares operational flood forecasting and early warning systems (FFEWSs) in transboundary river basins in Northwestern Europe, covering parts of Luxembourg, Germany, the Netherlands and Belgium. This region was hit by an extreme flood event in 2021 with over 200 fatalities. Expert interviews from the region revealed strong improvements of the FFEWSs after this flood event in all countries. All regions have invested in probabilistic flood forecasting systems to improve warnings, and all countries now use mobile phone-based alerts. The interviews also revealed that, while ensemble forecasting systems are well-developed, the translation of those meteorological and hydrological forecasts to impacts, warnings and actionable advices remains difficult. A main challenge is the operational implementation of impact-based forecasts and warnings. For example, interviewees highlighted the need for operational flood inundation predictions. However, Flanders is the only region where such forecasts are provided. Hydrological forecasts for smaller upstream tributaries are generally unavailable or subject to large uncertainties. Strong differences exist in flood warning levels and color codes across and within the countries. These differences can hamper information exchange between regions and institutions and may confuse crisis managers and the public. In response to the extreme flood event in 2021, Luxembourg and some regions of Germany have recently introduced an additional violet warning code for the most extreme weather and hydrological events. However, it is still under debate whether additional warning levels support more effective communication to the public and responders. It is strongly recommended to enhance forecasts with impact-based information, including maps delineating potential inundation areas and people and objects at risk. Such information can enable crisis managers and first responders to take more timely and appropriate actions.