The hydrological discharge (HD) model (Hagemann and Dümenil 1998; Hagemann and Dümenil Gates 2001) is used for the calculation of river runoff. The HD model has already been used for many years in the global Earth System Model (ESM) MPI-ESM (Giorgetta et al. 2013) and its predecessor ECHAM5/MPIOM (Roeckner et al. 2003; Jungclaus et al. 2006), but was also recently implemented into the regional ESMs ROM (Sein et al. 2015) and RegCM-ES (Sitz et al. 2017) as well as the regional climate model REMO-MPIOM (Elizalde 2011). The HD model was designed to run on a fixed global regular grid of 0.5° horizontal resolution, and it uses a pre-computed river channel network to simulate the horizontal transport of water within model watersheds. Originally, the HD model used a daily time step, but some refinements made during the MPI-ESM development allow sub-daily time steps, e.g. hourly. Recently, the HD model has been adapted to run on a regular grid of 5-Min. horizontal resolution, which corresponds to an average grid box size of 8-9 km. It can be run globally but it is also possible to set up the HD model for a region such as Europe. For the applications of the 5-Min. version over Europe, a regional domain was chosen that covers the land areas between -11°W to 69°E and 27°N to 72°N (Hagemann et al. 2020). Recently, the HD model has been equipped with the possibility to bias-correct discharges, which can be applied in standalone and coupled simulations (Hageman et al. 2024). The HD model is part of the GCOAST (Geesthacht Coupled cOAstal model SysTem) framework of Helmholtz-Zentrum Geesthacht where it was coupled to atmosphere and ocean compartments (GCOAST-AHOI, Ho-Hagemann et al. 2020; GCOAST-AHOI 2, Ho-Hagemann et al. 2024). In addition, the HD model code has been published on Zenodo (Hagemann and Ho-Hagemann 2021) and on GitLab at DKRZ. Moreover, the HD model became part of the ICON open source release 2024_10 where it is included as an external component (ICON Partnership 2024).

Figure 1 shows the structure of the HD model. It separates the lateral water flow into the three flow processes of overland flow, base flow, and river flow. Overland flow and baseflow are both represented by a single linear reservoir, and river flow is represented by a cascade of n equal linear reservoirs. Overland flow represents the fast flow component within a gridbox and uses surface runoff as input ,base flow represents the slow flow component within a grid box and is fed by drainage from the soil (or subsurface runoff) and the inflow from other grid boxes contributes to river flow. The sum of the three flow processes is equal to the total outflow from a grid box. The model parameters are functions of the topography gradient between grid boxes, the slope within a grid box, the grid box length, the lake area, and the wetland fraction of a particular gridbox. The model input fields of surface runoff and drainage resulting from the various climate or land surface model resolutions are therefore interpolated to the HD grid before fed into the HD model.

Figure 1. The HD model structure. The figure was designed by Ha Hagemann (pers. comm., 2021).

References

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