The Hydrological Discharge (HD) model The Hydrological Discharge (HD) model calculates the lateral transport of water over the land surface to simulate discharge into the oceans. It has been validated and applied in many studies since the publication of its original global 0.5° version (Hagemann and Dümenil, 1998; Hagemann and Dümenil Gates, 2001). Recently, a new version has been developed that can be applied globally at a 5 Min. (~8-9 km) resolution. This HD version was applied and validated over Europe by (Hagemann et al., 2020). The HD model has been coupled to several global and regional ESMs (cf. Hagemann et al., 2020). It separates the lateral water flow into the three flow processes of overland flow, baseflow, and riverflow. Overland flow and baseflow represent the fast and slow lateral flow processes within a grid box, while riverflow represents the lateral flow between grid boxes. The HD model requires gridded fields of surface and subsurface runoff as input for overland flow and baseflow, respectively, with a temporal resolution of one day or higher. Installation You have have to uncompress the tarfile hdmodel_5_0.tgz in your desired code directory and then follow the instructions in ./scr/readme_hdmodel.txt to modify and run the compile and run script in a linux/unix environment. In order to run the HD model, you need to download the HD model parameter files that are published as dataset on Zenodo.This dataset comprises parameter fields at 0.5° global resolution and at 5 Min. resolution (global, Europe). The script hdpara_cut.ksh cuts out a regional subdomain from the global 5 Min. HD parameter file (see readme_hdmodel.txt). Gridboxes that may laterally flow out of the domain at the regional boundaries are converted into sinks to avoid numerical exceedances of array bounds. Examples are provided for Europe and Australia. Further regional domains should be implemented analogously. Remark for coupling purposes Note that this HD model distribution should include everything that is necessary to run the standalone/offline version of the HD model. The HD model is also equipped with coupling capabilities using the OASIS coupler that can be enabled with the compiling Option -o ON (cf. readme_hdmodel.txt). Then, the HD model expects to receive input fields of surface runoff and drainage/subsurface runoff on the respective HD model resolution/domain. This means that an interpolation of these fields from the atmosphere/land resolution to the HD model domain has to be done by OASIS before. The interpolation of the discharge at the ocean inflow points (i.e. the river mouths) onto the ocean model grid is conducted within the HD model as OASIS is not able to thoroughly conduct such an interpolation of point-related fluxes (state of March 2021). In this respect, a coupling file is required for the HD model that is not part of this distribution. The program to generate such a coupling file is quite complicated and currently not in a very user friendly shape (this is work in progress). Hence, this is not part of this distribution.However, please feel free to contact Stefan Hagemann at Hereon (stefan.hagemann@hereon.de) who is open to cooperate and generate such a file for you. Copyright and Licenses Copyright 2021: Institute of Coastal Systems - Analysis and Modelling, Helmholtz-Zentrum Hereon HD model code is licensed under the Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0) Authors: Stefan Hagemann, Ha T.M. Ho-Hagemann Main Contact: Stefan Hagemann, Helmholtz-Zentrum Hereon, Email: stefan.hagemann@hereon.de Acknowledgements We acknowledge contributions of colleagues (see code headers) from the Max Planck Institute of Meteorology (MPI-M) and collaborating institutions who developed various infrastructure routines and who contributed to the implementation of the HD model into the coupled system ECHAM5/MPIOM and its successor MPI-ESM. We especially appreciated the help of Otto Böhringer (MPI-M) who separated the HD code from the fixed association with the MPI-ESM infrastructure. We are also grateful to Veronika Gayler (MPI-M) for her adapatations to Fortran90 and the implementation of various diagnostics. References Main publication for 5 Min. version of HD model (Vs. 4+): Hagemann, S., T. Stacke and H. Ho-Hagemann (2020) High resolution discharge simulations over Europe and the Baltic Sea catchment. Front. Earth Sci., 8:12. doi: 10.3389/feart.2020.00012. Further basic HD publications (Vs. 1, 0.5 degree) Hagemann, S., L. Dümenil (1998) A parameterization of the lateral waterflow for the global scale. Clim. Dyn. 14 (1), 17-31 Hagemann, S., L. Dümenil Gates (2001) Validation of the hydrological cycle of ECMWF and NCEP reanalyses using the MPI hydrological discharge model, J. Geophys. Res. 106, 1503-1510 First publication using coupled 5 Min. version of HD model over Europe Ho-Hagemann, H.T.M., Hagemann, S., Grayek, S., Petrik, R., Rockel, B., Staneva, J., Feser, F., and Schrum, C. (2020) Internal variability in the regional coupled system model GCOAST-AHOI, Atmos., 11, 227, doi: 10.3390/atmos11030227

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Note that the current run script creates a namelist that comprises two factors that are applied to two of the HD model parameters. This should only be done for the 5 Min. version. In a future version, this will be incorporated in the parameter file, so that the factors can be set to one in the run script.