Numerically quantifying the relative importance of topography and buoyancy in driving groundwater flow

Both topography and buoyancy can drive groundwater flow; however, the interactions between them are still poorly understood. In this paper, the authors conduct numerical simulations of variable-density fluid flow and heat transport to quantify their relative importance. The finite element modeling experiments on a 2-D conceptual model reveal that the pattern of groundwater flow depends largely upon the relative magnitude of the flow rate due to topography alone and the flow rate due to buoyancy alone. When fluid velocity due to topography is greater than that due to buoyancy at large water table gradients, topography- driven 'forced convection' overwhelms buoyancy-driven 'free convection'. When flow velocity due to buoyancy is greater than that due to topography at small water table gradients, mixed free and forced convection takes place. In this case, free convection becomes dominant, but topography- driven flow still plays an important role since it pushes the free convection cells to migrate laterally in the downhill direction. Consequently, hydrothermal fluid flow remains changing periodically with time and no steady state can be reached. The presence of a low-permeability layer near the surface helps eliminate the topography effect on the underlying free convection.