Where does the water table outcrop? Analysis of controlling factors across scales

Abstract:

The water table ‘roughly’ follows the topography. ‘Roughly’, because the water table intercepts the topography only in a few locations, such as along the gaining portions of streams and other surface discharge features, including drains, wetlands, seeps and the coastline. Elsewhere, the water table occurs below the land surface and is therefore controlled by recharge and hydraulic conductivity. Hence, the control of the topography on the water table arises through the occurrence of water table outcrops. This study analyses water table outcrops as a function of climatic, geologic and topographic factors in theoretical configurations, based on numerical simulations. The water table is treated as a free surface, whose position adapts to the parameters. The studied system is dimensionless and serves as a first-order approximation model for any scale. Varying the amplitude and frequency of topographic undulations, as well as recharge and hydraulic conductivity, relationships between key controlling factors and water table outcrops are investigated. The results provide fundamental understanding of water table shapes, and so of groundwater flow, under various conditions and without assuming the water table to be identical to the topography. Because water table outcrops also indicates areas of groundwater discharge and of interaction of groundwater with surface water, groundwater-dependent ecosystems and the atmosphere, the results have implications that go far beyond the scope of groundwater flow characterization. This includes the conceptualisation and modelling of stream baseflow, wetlands, riparian areas and water and energy exchanges between groundwater and the atmosphere.