lateral flow

Lateral flow at the soil bedrock interface (Weiler and McDonnell,
2003) illustrated in Figure 11, occurs in steep terrain with relatively
thin soil cover and low permeability bedrock, where water moves to
depth rapidly along preferential infiltration pathways and perches at
the soil-bedrock interface. Since moisture content near the bedrock
interface is often close to saturated, the addition of only a small
amount of new water (rainfall or snowmelt) is required to produce
saturation at the soil-bedrock or soil-impeding layer interface. Rapid
lateral flow occurs at the permeability interface through the transient
saturated zone. Once rainfall inputs cease, there is a rapid dissipation
of positive pore water pressures and the system reverts back to a slow
drainage of matrix flow.

The processes involved in the generation of subsurface stormflow by
groundwater ridging are illustrated in Figure 12. An idealized cross
section of a valley with a straight hillslope is shown. In a simplified
situation with uniform soils the water table has an approximately
parabolic form, and soil moisture content decreases with increasing
height above the water table. The shaded areas represent graphs of
soil moisture at the base, middle and near the top of the hillslope (a)
before the onset of rainfall; (b) as an initial response to rainfall; and
(c) after continuing rainfall. Because (in a) before the onset of water
input the water table slopes gently towards the channel there will be a
slow flow of groundwater to maintain the baseflow of the stream.
With the onset of surface water input, water that infiltrates near the
base of the hillslope will quickly reach the water table and cause the
water table near the stream to rise, early in a storm. Further upslope
the soil is dryer and distance to the water table greater. It therefore
takes longer for infiltrating water to reach the water table and where
the water table is deep all the infiltrating water may go into storage in
the unsaturated zone and not reach the water table for many days
after the storm. The initial response to water input is therefore as
depicted in Figure 12b, where the water table has risen near the
stream but remained unchanged further upslope. The rising water
table near the stream causes an increase in the hydraulic gradient
between the groundwater and stream, and increased subsurface flow
into the stream results. This is subsurface stormflow, and is
frequently seen to be groundwater that has been displaced by the
infiltrating water, and is thus old or pre-storm water bearing the

chemical and isotopic signature of water in the hillslope prior to the
storm, which may be different from the chemical and isotopic
signature of overland flow from rainwater that has not infiltrated.
Measurement of chemical and isotopic signatures of stream water,
ground water and rain water is commonly used in hydrology as a way
of inferring hillslope flow pathways. After continuing rain (Figure
12c), the water table has risen to the surface over the lower part of
the hillslope and the saturated area is expanding uphill. Some water
emerges from this saturated area and runs down slope to the stream.
This is termed return flow. Direct precipitation onto the saturated
zone (DPS) forms saturation excess runoff as described above
Figure 12. Groundwater ridging subsurface stormflow processes
in an area of high infiltration rate. (redrawn following Water in
Environmental Planning, Dunne and Leopold, 1978)
Figure 12 illustrates a region just above the water table that was close
to saturation. This is known as the capillary fringe, and can play an
important role in runoff generation in certain situations. Capillary
forces due to the surface tension between water and soil particles act
to pull water into the soil matrix above the water table and maintain
the capillary fringe at moisture content very close to saturation.