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

Abstract

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.

Energy considerations in groundwater‐ridging mechanism of streamflow generation

“…() and Waswa et al . (). The data used in the present study were obtained from the observation point U3 (Figure ), in the upper subcatchment, within a wetland area and close to a stream, a few metres in the upstream of the upper weir.…”

“…Some of the rainfall events of the summer season of 2000–2001 will be used in this study. Ninety‐six rainfall events occurred in that period, of which 11 were found to have caused groundwater‐ridging water‐table response (Waswa et al ., ; Waswa ). In the present paper, we will use six of those events, namely, event nos.…”

“…This additional potential energy raises the total potential energy above the tension (adhesion) energy, converting the vadose water in the tension saturation zone into phreatic water. There can be various means by which additional energy can be rapidly introduced into the capillary fringe without infiltration, but the most common natural ones are the Lisse effect and groundwater ridging, both of which are described by Waswa et al (2013). In groundwater ridging, the intense rainfall induces a proportionate amount of potential energy into the tension saturation zone, resulting in a proportionate rate and magnitude of a water-table rise.…”

Spatiotemporal Variability in Water Sources Controls Chemical and Physical Properties of a Semi‐arid Urban River System

“…δ 18 O values of groundwater and tributary inputs were indistinguishable because groundwater can contribute to surface water flow in tributaries (Becker ; Waswa et al. ; Kormos et al. , b; Brooks et al.…”

Quantify the Pore Water Velocity Distribution by a Celerity Function

“…The air entry pressure 7 Geofluids values in the modified Mualem-van Genuchten model were adopted from those of the Brooks-Corey model for all 5 typical soils and listed in the last column of Table 2. Solely using either specific discharge or average pore water velocity to quantify the transport processes is theoretically insufficient [20,22,81]; we included the celerity and kinematic ratio in Table 2 to provide a more complete illustration of the dependence of pore water flow velocity and pressure propagation on soil effective saturation.…”