Geoscience Reference
In-Depth Information
and low slope areas, which will tend to stay wet during recession periods. The area of saturated soil
will tend to expand with increased wetting during a storm and reduce again after rainfall stops at a rate
controlled by the supply of water from upslope. This is the
dynamic contributing area
concept. Any
surface runoff on such a saturated area may not all be due to rainfall but may also be due to a return flow
of subsurface water. In this way, surface runoff may be maintained during the period after rainfall has
stopped. When overland flow is generated, by whatever mechanism, some surface
depression storage
may need to be satisfied before there is a consistent downslope flow. Even then, surface flow will tend to
follow discrete pathways and rills rather than occurring as a sheet flow over the whole surface.
A similar concept may be invoked in areas where responses are controlled by subsurface flows. When
saturation starts to build up at the base of the soil over a relatively impermeable bedrock, it will start to flow
downslope. The connectivity of saturation in the subsurface will, however, initially be important. It may be
necessary to satisfy some initial bedrock depression storage before there is a consistent flow downslope.
The dominant flow pathways may be localised, at least initially, related to variations in the form of
the bedrock surface (McDonnell
et al.
, 1996). Some catchments, with high infiltration capacities and
reasonably deep soils, may have responses dominated by
subsurface stormflow
. It is worth remembering
that a 1 m depth of soil, with an average porosity of 0.4 has a storage capacity of 400 mm of water.
Thus, if the infiltration capacity of the soil is not exceeded, a large 100 mm rainstorm could, in principle,
be totally absorbed by that 1 m soil layer (ignoring the effects of any downslope flows), even if the
antecedent storage deficit is only a quarter of the porosity. It has further been suggested that a certain
degree of antecedent wetness is required before some threshold of connectivity is reached and significant
downslope flow is achieved (the “fill and spill” hypothesis, see Tromp van Meerveld and McDonnell,
2006). Some soils are susceptible to piping for both natural and anthropogenic (field drainage) reasons.
In the right conditions, such pipes can provide rapid conduits for downslope flows (see Jones, 2010;
Chappell, 2010).
It is a common (and very convenient) assumption that the bedrock underlying small upland catchments
is impermeable. This is not always the case, even in rocks that have little or no primary permeability in
the bulk matrix. The presence of secondary permeability in the form of joints and fractures can provide
important flow pathways and storage that may be effective in maintaining stream baseflows over longer
periods of time. It is very difficult to learn much about the nature of such pathways; any characteristics are
often inferred from the nature of recession curves and the geochemistry of baseflows since the bedrock
can provide a different geochemical environment and long residence times can allow weathering reactions
to provide higher concentrations of some chemical consitutuents (see, for example, the study of Neal
et al.
, 1997 in the Plynlimon research catchments in Wales).
There is an interesting possibility that connected fracture systems that are full of water could act as
pipe systems, transmitting the effects of recharge very rapidly. Remember that if water is added to one
end of a pipe full of water, there will be an almost instantaneous displacement of water out of the other
end, whatever the length of the pipe and even if the velocities of flow in the pipe are relatively slow.
The reason is that the transmission of the pressure effect of adding the water is very much faster than
the actual flow velocity of the water. Such displacement effects are an explanation of rapid subsurface
responses to storm rainfalls (see Section 1.5).
The perceptual model briefly outlined above represents a wide spectrum of possible hydrological
responses that may occur in different environments or even in different parts of the same catchment
at different times. Traditionally, it has been usual to differentiate between different conceptualisations
of catchment response based on the dominance of one set of processes over another, for example, the
Hortonian model
in which runoff is generated by an infiltration excess mechanism all over the hillslopes
(Figure 1.4a). This model is named after Robert E. Horton (1875-1945), the famous American hydrologist
(he may be the only modern hydrologist to have a waterfall named after him) who worked as both
hydrological scientist and consultant. I am not sure that he would have totally approved of such widespread
use of the infiltration excess concept. Although he frequently used the infiltration excess concept as a
