On the benefits of new, seamless prediction products in operational hydro-meteorological ensemble forecasting in small catchments

In recent years, small-scale heavy precipitation in Saxony has repeatedly led to exceptional flood situations in small and very small catchment areas in the low mountain range. Such catchment areas often react very quickly to precipitation events. Due to the sometimes tremendous effects, particularly in residential areas, early flood warning is essential. The forecast and the associated flood warning enable local authorities, emergency services and water authorities to prepare for the potential flood situation at an early stage and initiate the necessary measures.

We developed a flood early warning system that is in operational use in three pilot regions in Saxony. Data processing is event-driven and is controlled by a so-called sentinel component based on meteorological forecasts. This sentinel checks every 30 minutes whether a specific precipitation threshold in the forecast for the next 24 or 48 hours will be exceeded at any location in Saxony. In this case, the necessary precipitation data for a hydrological ensemble forecast is compiled. Two demonstrators were implemented for this purpose: (1) use of precipitation for observation and forecasting from established products of the German Weather Service (RADOLAN-RW – radar based QPE, RADOLAN-RV – radar based nowcasting, ICON-D2-EPS – ensemble QPF) and (2) precipitation from new, prototype products of the German Weather Service (pyRADMAN, SINFONY-INTENSE). With the pyRADMAN product, radar calibration is carried out by assimilation to rain gauges and commercial microwave links with a temporal resolution of 15 minutes. The SINFONY-INTENSE product integrates nowcasting and forecasts data into a seamless prediction ensemble with 21 members. Based on the respective combination of data, a separate decision is made for each warning region in Saxony as to whether a specific flood situation is imminent. If the system recognises a potential flood situation, the catchments in the according warning region are simulated using the event-based hydrological model DeHM. DeHM includes processes for runoff formation, runoff concentration, routing and simulation of dams. Data assimilation is carried out using online coupling with runoff data at gauging stations and reservoir levels in flood retention basins or dams.

The demonstrator with established products has been running since August 2023 and the demonstrator with prototype products since December 2023. The performance of both systems is evaluated. Parameters such as KGE or Percentage Error in Peak alongside threshold-based parameters such as False Alarm Ratio or Area under ROC Curve (AUC) allow the quality of both demonstrators to be assessed using various prediction ranges. The differences between the two demonstrators are shown on the basis of the quality measures and specific simulation results, and the associated benefits for early flood warning are discussed.