## 3 Model Development
### Coupling with a Hydrodynamic Model  
River discharge is a key component of the global water cycle, integrating meteorological and hydrological conditions on multiple temporal and spatial scales. Weather forecast uptake in hydrological forecasting [54] is a growing area, and developments by the climate modeling community to enhance the representation of the Earth system have motivated a tighter integration of atmospheric models and the surface water balance [55]. River discharge provides a spatially integrated measure of the surface water budget and can be used to evaluate forecast skill [56]. Furthermore, in atmosphere-ocean coupled numerical weather prediction, freshwater inflow to the ocean can impact ocean circulation [PERSON] et al. [57]. Therefore, the representation of rivers and their role on the ocean and land water is relevant in several applications, motivating the integration of the Catchment-based Macro-scale Floodplain model (CaMa-Flood) into the ECLand system. CaMa-Flood model was developed to simulate continental-scale river hydrodynamics. The global river network (excluding Antarctica) is discretized in irregular unit-catchments, representing hydrological units. Sub-grid topographic parameters describing the river channel and floodplains profile are used to diagnose flooded area and water levels from water storage. The irregular-shaped unit-catchments are assigned to a rectangular grid, providing a \"grid-vector hybrid river network map\". This approach allows a realistic representation of sub-grid topography while keeping data handling simple. River flow velocity is computed by the local inertial equation [58]. There is also an option to compute flow velocity using a kinematic wave approximation. The only prognostic variable, water storage, evolves in time according to the input runoff and upstream inflow and downstream outflow at each unit-catchment. The detailed description of CaMa-Flood can be found in the description papers [59; 60; 61]. Figure 7: Lake cover difference: new GSWE based map—operational map. The key characteristic of CaMa-Flood is the explicit representation of flood stage, representing water level and inundation extent, which complements the conventional use of river discharge [62]. This also allows exploring two-way coupling with the land surface model [63]. The baseline topography is based on MERIT DEM/MERIT Hydro [64; 65]. River channel width is objectively parametrized using satellite measurement [66; 67]. The CaMa-Flood source code is integrated in the ECLand system allowing stand-alone CaMa-Flood simulations or simulations with 1-way and 2-way coupling between ECLand and CaMa-Flood. The coupling is performed sequentially and the code is integrated as a single executable (without external couplers) following a similar approach as the coupling with the ocean model to IFS [68]. In 1-way coupling, the default configuration, surface and sub-surface runoff are sent to CaMa-Flood. In the stand-alone mode CaMa-Flood reads from input files the runoff data, reproducing the 1-way coupling results. This configuration has a lower computational cost, allowing for testing of the hydrodynamic model as well as driving the model with other runoff data sources like reanalysis. The 2-way coupling between ECLand and CaMa-Flood was also experimentally developed allowing initial testing of the interaction between inland water in ECLand and flooded areas in CaMa-Flood. Inland water evaporation (in ECLand) can be extracted from flooded areas in CaMa-Flood, and the flooded area extent can update the inland water fraction in ECLand. CaMa-Flood integration in the ECLand system is only available in offline mode supporting shared memory parallelism via OpenMP. A full integration of CaMa-Flood in the IFS is envisaged in upcoming cycles to support coupled atmosphere-ocean-land-rivers simulations aiming at a full representation of the water cycle. This will still require developments both at the level of distributed memory parallelism in CaMa-Flood and in the coupling of river discharge with the ocean model.