Understanding topography-driven groundwater flow using fully-coupled surface-water and groundwater modeling

Numerical models with spatially-varying head as top boundary conditions were used in previous studies to understand topography-driven groundwater flow. The head boundary conditions could cause artifacts of extremely large, but unrealistic recharge rates owing to unlimited supply of water. This study adopted a fully-coupled surface–subsurface hydrologic modeling approach to simulate transient topography-driven groundwater flow and also surface-water flow under homogeneous and isotropic settings. Two 100-year climate datasets and five hydraulic conductivities (K, 0.01–100 m/d) were tested in numerical experiments. In the base case with a wet climate (annual precipitation 1696 mm/y) and K of 1 m/d, groundwater head at two different locations close to both lateral boundaries fluctuates only within 5.1 m and 9.6 m, respectively, during the 100-year period. Despite the local water table fluctuations caused by the variability in the climatic record, large-scale groundwater flow systems can be assumed in dynamic equilibrium provided stationary climate. Long-term average exchange fluxes are spatially constant and limited by precipitation infiltration when surface water is absent, whereas they vary from positive to negative values (i.e., recharge to discharge) spatially when surface water is present. Sensitivity analysis suggests that wetter climate and smaller K lead to more inundation of the land surface, stronger hierarchical nesting of groundwater flow systems and more variable exchange fluxes. Overall, our first fully-coupled modeling of topography-driven groundwater flow implies that attention must be paid to causality between head and flow, and climatic record as boundary conditions may be more appropriate due to its relaxed manner.