Geoscience Reference
In-Depth Information
Fig. 11.11 Schematic illustration of the
rapid subsurface storm flow
(SF) through various types of
preferential flowpaths,
pipes and macropores. The
relative amounts of new
(dashed arrows) and old water
(solid arrows) in the mixing
process depend mainly on the
precipitation intensity and on
the pre-storm soil moisture
conditions.
I n f i l t r a t i o n
P
B e d r o c k
SF
(McGlynn
et al
., 2002) on the basis of a fifth set of hillslope-scale tracer measurements
with bromide at the same catchment. The main conclusion from that study was that it
is not the surface topography, but rather the spatial pattern of the bedrock topography,
with local preferential flow and mobile and immobile regions, conditioned by small
local depressions in the bedrock, which controls the tracer outflow variability; tracer
material and old water may remain trapped temporarily in such depressions and become
mobilized only by a new storm event.
From measurements in a forested Canadian Shield basin in Ontario, Peters
et al
.
(1995) concluded that preferential flow channels brought the water vertically down,
after which it flowed laterally over the bedrock and that practically all the lateral flow
occurred withinathin weathered zone near the soil-bedrock interface. The conductivity
of this preferential flow layer appeared to be so large that some of the fast flows and
peak runoff were suspected to be of the non-Darcy type. The storm runoff in the channel
consisted of a mixture of event and pre-event water. Thiswasinterpreted to show that
the fast infiltration of the event water caused saturated soil conditions above the bedrock,
which in turn resulted in the downhill flow of both event and pre-event water; moreover,
during the hillslope transport, there was ample opportunity for interaction between the
event runoff water and the soil matrix.
In summary, the subsurface stormflow, observed in several of the hillslope experiments
reviewed here, exhibited the common feature of unimpeded entry by new water from
rainfall into the soil, followed immediately by rapid downslope flow through preferential
paths, pipes and other macropores; thisflowinvolved mixing with the old water already
present in the soil profile (see Figure 11.11), to varying degrees depending on the intensity
of the rain and on the initial moisture status of the soil mantle.
Throughflow in a shallow permeable layer
In many catchments covered with natural vegetation the soil mantle has a relatively
permeable top layer consisting of organic debris and mineral soilwith high organic
content; typically, this layer has a thickness of only a few tens of centimeters and its
bottom interface is characterized by an abrupt decline in hydraulic conductivity in the
underlying mineral soil. Thus infiltrating rainwater tends to flow and build up along
