Transient Pressure Waves in the Vadose Zone and the Rapid Water Table Response

The behavior of a water table (WT) is important in understanding groundwater dynamics. Results are presented of a disproportionate response of a WT in two distinct transient pressure wave mechanisms that occurred during rainfall events of the 2000-2001 summer season and in two different hillslope zones in the Weatherley research catchment of South Africa. The first mechanism was a groundwater ridging pressure wave, which frequently occurred at the low-lying wetland zone and when the capillary fringe was close to the ground surface. Results from this zone indicated that groundwater ridging WT responses were caused by rainfall events that had a threshold total rainfall of 10 mm, and the magnitude of the responses had a linear relationship with the peak rainfall intensities. The mechanism, which exhibited the characteristics of the Lisse effect (pneumatically pressurized WT response), occurred at an elevated zone of the catchment, where bedrock is overlain by a shallow soil profile and perched groundwater. This second mechanism was particularly evident during a rainfall event that occurred in the early part of the season. During this event, a peak rainfall intensity of 228 mm/h generated a pressure wave from the ground surface toward the WT, where it disproportionately elevated the hydraulic head in the groundwater by 106 cm of H2O at the toeslope and without groundwater recharge via the infiltration profile. A laboratory experiment was performed and verified that the second mechanism was the Lisse effect and that, as in the groundwater ridging, the capillary fringe played a significant role.