Abstract. The subglacial hydrological system affects the motion of ice sheets, the ocean circulation by freshwater discharge, as well as marginal lakes and rivers. For modelling this system a porous medium model has been developed, representing a confined-unconfined aquifer system (CUAS) with evolving transmissivity. To allow for realistic simulations, we developed CUAS-MPI, an MPI-parallel C/C++ implementation, which employs the PETSc infrastructure for handling grids and equation systems. We describe the CUAS model and our software design and validate the numerical result of a pumping test using5 analytical solutions. We then investigate the scaling behavior of CUAS-MPI and show, that CUAS-MPI scales up to 3840 MPI processes running a realistic Greenland setup. Our measurements show that CUAS-MPI reaches a throughput comparable to the throughput of ice sheet simulations, e.g. the Ice-sheet and Sea-level System Model (ISSM). Lastly, we discuss opportunities for ice-sheet modelling, future coupling possibilities of CUAS-MPI with other simulations, and consider throughput bottlenecks and limits of further scaling.
<p>Simulating the hydrological systems underneath ice sheets and glaciers is important for estimating the freshwater flux into the ocean as well as inferring the characteristics of the hydrological system and its impact on ice sheet dynamics. In particular, simulations of the subglacial hydrological system in high temporal and spatial resolution and coupled to ice sheet models are needed to investigate the formation of ice streams. In order to be able to run simulations efficiently, both codes need to be parallelised. To this end, we present our approach for a parallelised version of the confined-unconfined aquifer system (CUAS) model (Beyer et al., 2018) that was established as a python code. CUAS is simulating an effective porous medium layer, in which the transmissivity indicates if the flow is channelised. Transmissivity is evolving by melt, creep and cavity opening. A fully implicit finite difference scheme is used for the hydraulic head while an explicit Euler time step is used for the transmissivity.&#160;</p><p>The new CUAS-MPI version is written in C++ and instrumented for performance measurements. The parallelisation is done with MPI, where we take advantage of PETSc data structures and linear equation system solvers. The code has been designed to be coupled to the Ice Sheet and Sea-level system Model (ISSM) using preCICE (precice.org).&#160;</p><p>Pumping tests that are widely used in applied groundwater hydrology are performed to test the model implementation including the boundary conditions and to compare with the analytical solutions. We further present test applications to the Greenland Ice Sheet, with the major focus on performance, rather than on characteristics of the hydrological system.</p>
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