## 4 Evaluation of Model Developments
### Evaluation of Coupled River Discharge  
The evaluation of river discharge in different configurations is presented in Figure 24. The main goal of these results is to illustrate the different capabilities of the system for various applications. The evaluation displays the distribution of the temporal correlation and percent bias of daily river discharge for several stations on the Mackenzie (panels a,b) and Danube (panel c,d) basins, with the number of stations listed on the top of each panel. The observations are taken from the Global Runoff Data Centre (GRDC). All simulations were carried out at the \(1/4^{\circ}\) resolution for CaMa-Flood and TL639 for ECLand (same resolution as ERA5, about 32 km) extending since 1979 to 2018, and the evaluation is performed between 1982 and 2018 considering the available station data with at least 5 years of daily data. The boxplots in each panel show the distribution of the correlation and percent bias for six configurations. \"coup1\" is the default 1-way coupling with ECLand driven by ERA5 near-surface meteorology. \"coup2\" is a 2-way coupling simulation allowing water extraction from floodplains and update of inland water fraction in ECLand following CaMa-Flood flooded area extent. \"iner\" and \"kine\" are two stand-alone CaMa-Flood simulations driven by the runoff of \"coup1\", differing in the flow velocity calculations in CaMa-Flood, using the local inertia in \"iner\" (the default configuration) and the kinematic wave in \"kine\". Finally, \"era5\" and \"era5l\" are also stand-alone CaMa-Flood simulations driven by the runoff of the reanalysis ERA5 and ERA5-Land, respectively. Figure 23: A 2 m temperature normalised root mean square error (RMSE) difference for northern hemisphere Summer between 4 dvar simulations with the updated surface water map and the current operational version. The error differences refer to 36-h (**a**) and 48-h (**b**) forecasts initialized at OUTC. The results in Figure 24 show a neutral impact on river discharge simulation of the 2-way coupling, when compared with 1-way coupling on the selected basins. This is expected as both basins do not have large flooded areas. Results on some stations in the Amazon basin show some impact but reduced (not shown). However, the 2-way coupling configuration is still experimental and further developments are necessary for a consistent 2-way coupling, in particular on the handling of grid-scale versus sub-grid scale inland water in ECLand. The CaMa-Flood stand-alone configuration iner, shows identical results to coup1, as expected. The kinematic solution for flow velocities (kine) has some impact on the temporal correlations, with mixed results in the two analyzed basins. The local inertia solution is more accurate for large upstream areas where small water level slopes dominate the river flow, and is of particular relevance in representing surface water elevation. In smaller upstream regions with higher terrain slopes both local inertial and kinematic wave solutions are similar, with a reduced computational cost for the kinematic wave solution. Therefore, for particular applications were computational cost is relevant (e.g., ensemble forecasting) this could be considered. Finally, the flexibility of driving the hydrodynamic component with different runoff inputs is illustrated with the results of using ERA5 and ERA5-Land (era5 and era5l simulations in Figure 24). The era5l simulations are very close to coup1 (and iner) as the main difference is the spatial resolution of the runoff fields produced by ECLand (9 km in the case of era5l and 35 km in coup1/iner). Larger differences appear in era5, in particular for the percent bias (Figure 24b,d). These are associated with the soil moisture and snow mass increments of the land data assimilation in ERA5 that changes the land surface water balance [56].