Enhancing global water cycle representation through two-way coupling of VIC-WUR and MODFLOW 6

Abstract. Groundwater is a critical component of the global water cycle, sustaining streamflow during dry periods and supporting evapotranspiration where water tables are shallow. However, in many global hydrological models, groundwater processes are represented in a simplified way, which limits the simulation of water tables, lateral flow, groundwater–river exchange, and capillary rise. Here, we present VIC-WUR v3.0, a new global hydrological modelling framework in which the land-surface model VIC-WUR is coupled with MODFLOW 6, a physically based groundwater flow model. Compared with many existing global hydrological models, the new framework explicitly simulates transient groundwater flow and introduces a groundwater-depth-dependent capillary rise parameterisation constrained by soil hydraulic properties.

The coupled model is applied globally at 5 arcmin resolution under naturalised conditions for 1980–2009. The coupled model results are broadly consistent with the large-scale behaviour reported by other global hydrological and groundwater modelling studies. Groundwater recharge falls within the range of existing global model estimates, while VIC-WUR v3.0 tends to simulate comparatively higher recharge. Simulated capillary rise is spatially concentrated rather than spatially widespread, emerging mainly in regions with shallow groundwater where it can support soil moisture and evapotranspiration. Relative to the uncoupled VIC-WUR configuration, the coupled simulation also improves the representation of seasonal river discharge dynamics, particularly under low-flow conditions. These results show that VIC-WUR v3.0 provides a more physically based representation of groundwater–surface water interactions at the global scale and highlights the importance of capillary rise in shallow-groundwater environments. The model offers a new platform for investigating water availability under future human impacts, climate change, and land-use change.