UC Berkeley

The response of unconfined aquifers to Earth's solid tides (Earth tides) has been used to study a number of hydrogeological, environmental, and ecological processes. The traditional interpretation of such responses has been based on a model that assumes free flow of groundwater to the water table, even though capillary effects have long been recognized to influence the response of coastal aquifers to ocean tides. Here we study the capillary effect on the response of unconfined aquifers to Earth tides, using a numerical procedure that couples groundwater flow and poroelastic strain in both the saturated and the unsaturated zones. We show that when the water table rises to the surface and the capillary zone disappears, the predicted tidal response is identical to that predicted by the traditional model. But when the capillary zone is present when the water table falls below the surface, the predicted amplitude ratio and phase shift depart notably from those predicted by the traditional model. We apply the model to a field case where large seasonal changes of tidal response have been documented. We show that the large seasonal changes in the tidal response may be due to the change of the water table from a capillary-affected state to a capillary-free state when the water table rises annually to the ground surface. Thus, the omission of the capillary effects in the interpretation of the response of unconfined aquifers to Earth tides may lead to incorrect inference of subsurface processes and properties.