Global groundwater modeling: Proof-of-concept of 3D variably saturated flow simulation at kilometer resolution

• Integrated hydrologic modeling. • Global simulations. • Kilometer resolution • Exascale high-performance computing readiness. The proof-of-concept study presents, for the first time, the results of integrated numerical hydrological simulations at kilometer resolution on a global scale. Using available datasets and applying significant simplifications to the real terrestrial system, the model was informed with hydrofacies, soil texture and topographic slopes, and effective recharge at the upper boundary. A steady-state spin-up was performed, resulting in a 3D pressure head distribution of the water continuum from 60 m deep variably saturated groundwater to surface water. Relative saturation and diagnostic water table depth were examined, resolving variability over several orders of magnitude. In our opinion, the added value of the partial differential equation (PDE) based simulations outweighs the computational resources required, which are considerable. These simulations are possible, because of the advent of massively parallel, accelerator based supercomputer architectures and performance portable scientific software. While the current simulation results may not be reliable from the perspective of stakeholders at this stage of model development, the study demonstrates the feasibility of prognostic groundwater simulation at the global scale, and will stimulate future model improvements, including the quantification of uncertainties. Simultaneously, the study opens new avenues for future research in the context of hyper-resolution global Earth system modelling.