The 20-year history of HEPEX - Enhancing hydrological forecasting through strategic innovations

Creating forecast systems that add value across spatial scales and time horizons is crucial for a variety of fields, from meteorology and climate science to business and public policy. The priorities for developing such systems may vary depending on the specific domain and objectives. Over the past 20 years, the international community of practice known as the Hydrological Ensemble Prediction Experiment (HEPEX) has been seeking to advance the science and practice of hydrological ensemble prediction and its use in impact- and risk-based decision-making. HEPEX is a volunteer-based community, active since 2004, with over 600 members (hepex.org.au/). It has been promoting knowledge utilizing cutting-edge techniques and data to innovate hydrological forecasting methods, products and systems, and improve services for users in the water-related sectors. Here, we present an overview of the history of HEPEX and reflect on the key priorities recently proposed by the community for (co-) creating hydrological forecast systems that add value across spatial scales and time horizons. We highlight that hydrological forecasts have advanced through rigorous data management that incorporates diverse, high-quality data sources and the application of cutting-edge AI/ML techniques to improve predictive accuracy. HEPEX has played a critical role in enhancing the reliability of water management globally by standardizing ensemble forecasting and fostering a broader framework for forecast evaluation. This complements HEPEX's broader initiative to bridge the gap between research-to-operations practice, making forecasting solutions both practical and accessible. Finally, we highlight how efforts supporting the United Nations Early Warnings for All initiative can contribute to the development of robust early warning systems through extensive global training and the sharing of technology. The integration of advanced science, user-centric methods and global collaboration can provide a solid framework for improving the prediction and management of hydrological extremes, aligning forecasting systems with the dynamic needs of water resource management in a changing climate.