Operational Earth system hydrology at ECMWF

Abstract. The representation of river routing and floodplain dynamics remains a key limitation in many global Earth system and numerical weather prediction models, despite their importance for hydrological extremes, water resources, and land–atmosphere interactions. This paper presents the implementation of a global hydrodynamic river routing capability within the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) through the integration of the Catchment-based Macro-scale Floodplain (CaMa-Flood) model into the ecLand land surface scheme.

The system enables the routine simulation of river discharge, water level, and inundation extent within a physically consistent Earth system modelling framework. We describe the scientific design and technical developments required to transition from research to operational implementation, including coupling strategies, state initialisation, parallelisation using a basin-based hybrid MPI/OpenMP approach, and integration within ECMWF's Research-to-Operations workflow. The implementation maintains consistency across multiple configurations, from offline experiments to fully coupled forecasts, while preserving operational constraints such as computational efficiency and forecast stability.

The added value of the integrated system is demonstrated through a range of applications, including global water resources monitoring contributing to the World Meteorological Organisation State of Global Water Resources Report, real-time ensemble flood forecasting, high-resolution regional simulations, and land-surface diagnostics. Results illustrate that river discharge provides a powerful integrative constraint on the land water cycle and supports the identification of model deficiencies in hydrological processes. The system also enables the generation of continuous, global, high-frequency hydrological datasets suitable for both operational applications and emerging data-driven modelling approaches.

This work establishes a new capability for global hydrological prediction within an operational Earth system model and provides a framework for future developments toward fully coupled atmosphere–land–river interactions.