*2.2. Watershed Modelling*

The MOHID Land model is the hydrological model used to estimate river flow and water temperature for the study region. It is a physically based, spatially distributed, continuous, variable time-step model for inland water property cycles [17]. The model is based on finite volumes organized into a structured grid, rectangular in the horizontal plane, and z-level geometry in the vertical direction. It includes four compartments or media (atmosphere, porous, soil surface, and river network). Water moves through the mediums based on mass- and momentum-conservation equations. The atmosphere compartment provides varying space and time surface boundary conditions (precipitation, solar radiation, wind, etc.). The surface layer is described by a 2D horizontal grid, while the porous media consists of a 3D domain of variable-layer thickness that shares the horizontal grid with the surface layer. The river network is a 1D domain defined from the digital terrain model (DTM), with watershed reaches linking surface-cell centres. Fluxes are computed through the finite volumes' sides, and state variables are computed at the cell centres to ensure transported properties are conserved. The model uses a dynamic time step that increases during dry periods and reduces as water fluxes increase (e.g., during rain events). The MOHID Land model can simulate single-catchment or multi-catchment domains. Although MOHID Land can be configured to simulate water properties such as oxygen, suspended sediment, and nutrient concentrations, in the LAMBDA project the simulations focused on obtaining river flows and water temperature.

The main objective of the watershed modelling simulations was to produce regionalscale water budgets covering the land boundary of the two CMEMS MFCs. For the LAMBDA project, the drainage basins draining the study area were divided into ten domains using regular grids with a horizontal resolution of 0.05◦ (Figure 1). Only the Loire (France) and the Severn (UK) rivers were simulated with higher resolutions due to developments during the Hazrunoff project (http://www.hazrunoff.eu/, accessed on 15 April 2022). In general, hydrologic models with a horizontal scale of 1–9 km resolution can obtain similar results and perform well for storm events [18]. The watershed and land-use grids were populated with the EU 30 m Digital Elevation Model (DEM) and Corine Land Cover 2012 obtained from the Copernicus Land Monitoring Service (https:// land.copernicus.eu/, accessed on 15 April 2022), respectively. 3D soil hydraulic properties were obtained from Tóth et al. [19], while channel cross-sections were defined with the database from Andreadis et al. [20]. Each domain was simulated for the 2008–2019 period with meteorological conditions calculated with the ERA5 reanalysis product (horizontal resolution 31 km) from the Copernicus Climate Change Service (https://climate.copernicus. eu/, accessed on 15 April 2022).

**Figure 1.** *Cont*.

**Figure 1.** Topography and drainage network of the LAMBDA project watershed domains. The LAMBDA watershed domains are: (**a**) Western Iberian Peninsula domain; (**b**) Western France domain; (**c**) Seine River domain; (**d**) Somme, Scheldt, and Meuse rivers domain; (**e**) Rhine River domain; (**f**) Northwest Germany domain; (**g**) Elbe River domain; (**h**) Denmark domain; (**i**) Southern Norway domain; (**j**) United Kingdom and Ireland domain.
