| dc.description.abstract | Previous studies found that during a rainfall event, pre-event water, which exists in the
catchment before the event, may appear in significant amounts in the stream stormflow
hydrograph. Pre-event water is predominantly groundwater. Among the mechanisms that
have been proposed to explain the rapid mobilization of pre-event water from hillslopes are:
(1) groundwater ridging (GWR) i.e. the rapid rise of a water table in environments, where
the capillary fringe, or the zone of tension saturation, is very close to the ground surface and
(2) the Lisse Effect (LE) i.e. the rapid response of a groundwater level to pressurized pore
air in the unsaturated zone. Published literature explains that GWR is caused by the
application of a small amount of water on the ground surface. On the LE, it is explained that
pressurized pore air acts at the water table, resulting in a rapid rise of the water level in a
well, screened below the water table. These explanations are insufficient on the physical
processes involved in GWR and the LE. The objectives of this study were: (1) to use the
commonly observed catchment hydrological processes i.e. tensiometric pore water pressure,
shallow groundwater levels, rainfall data and the hydraulic properties of soils, to quantify
and describe the physical processes involved in GWR and the LE mechanisms; (2) to
perform laboratory experiments, in order to understand the physical processes involved in
the LE; and (3) to develop a mathematical theory that can describe the physical processes in
the LE. Results indicated that GWR and the LE are caused by the addition (elevation) of
potential energy in water within the capillary fringe. In GWR, the additional energy is from
the intense rainfall. In the LE, the additional energy is from compressed pore air in the
unsaturated zone. In both mechanisms, the added energy diffuses through the capillary
fringe, as a downward pressure wave, releasing the tension forces in water. As soon as the
downward pressure wave-front arrives at the water table, the water table begins to ascend, as
an upward pressure wave. The ascending water table steepens the hydraulic gradient, which
results in the rapid groundwater fluxes, without the recharge of the water table by the
infiltration profile. | en_US |
