Performance Evaluation of a Real-time Regional Ensemble Flow Forecasting System in Catalonia: Accuracy, Robustness, and Timeliness

Real-time ensemble flow forecasting systems are increasingly employed to manage hydrological risks associated with extreme weather events and a changing climate. The effectiveness of flow forecasting systems hinges on their ability to deliver accurate, robust, and timely predictions, despite inherent uncertainties in their components. The assessment of the system’s performance after a long-term application is essential to establish its value as a reliable reference tool for stakeholders in hydrological management and decision-making.

This study presents an approach to evaluate the performance of a regional ensemble flow forecasting system operational in real time since June 2020 over the region of Catalonia (NE Spain). The system generates flow forecasts for all gauging stations managed by the Catalan Water Agency using a modified version of the HBV rainfall-runoff model, with rainfall inputs combining QPE (blending radar and rain gauge observations) with the 52-member ensemble precipitation forecasts produced by the European Centre for Medium-range Weather Forecast (ECMWF). The analysis spans a four-year period and focuses on significant rainfall events, enabling a comprehensive analysis of the system’s accuracy, robustness, and timeliness as a function of lead time. Both deterministic metrics and probabilistic scores are applied to evaluate the quality of the flow forecasts. The evaluation focuses on assessing the impact of different sources of uncertainty using different discharge references: (1) observed flow; (2) flow simulated with the full series of rainfall observations; (3) flow simulated with no-rainfall forecasts. Additionally, key sources of uncertainty, such as rainfall variability, catchment response and forecast initialization errors, are identified to inform targeted system enhancements and improve forecast reliability.