Geoscience Reference
In-Depth Information
be the case. In Chapter 4 the relationship between
groundwater and streamflow has been explained (see
Figure 4.9). However in general it can be said that
baseflow is provided by the slow seepage of water
from groundwater into streams. This will not neces-
sarily be visible (e.g. springs) but can occur over a
length of streambank and bed and is only detectable
through repeated measurement of streamflow down
a reach.
Summary of storm runoff mechanisms
The mechanisms that lead to a storm hydrograph
are extremely complex and still not fully under-
stood. Although this would appear to be a major
failing in a science that is concerned with the
movement of water over and beneath the surface, it
is also an acknowledgement of the extreme diversity
found in nature. In general there is a reasonable
understanding of possible storm runoff mechanisms
but it is not possible to apply this universally.
In some field situations the role of throughflow and
piston flow are important, in others not; likewise
for groundwater contributions, overland flow and
pipeflow. The challenge for modern hydrology is
to identify quickly the dominant mechanisms for
a particular hillslope or catchment so that the under-
standing of the hydrological processes in that
situation can be used to aid management of the
catchment.
The processes of storm runoff generation described
here are mostly observable at the hillslope scale.
At the catchment scale (and particularly for large
river basins) the timing of peak flow (and con-
sequently the shape of the storm hydrograph) is
influenced more by the channel drainage network
and the precipitation characteristics of a storm than
by the mechanisms of runoff. This is a good example
of the problem of scale described in Chapter 1.
At the small hillslope scale storm runoff genera-
tion mechanisms are important, but they become
considerably less so at the much larger catchment
scale.
Channel flow
Once water reaches the stream it will flow through
a channel network to the main river. The controls
over the rate of flow of water in a channel are to
do with the volume of water present, the gradient
of the channel, and the resistance to flow experi-
enced at the channel bed. This relationship is
described in uniform flow formulae such as the
Chezy and Manning equations (see p. 92). The
resistance to flow is governed by the character of the
bed surface. Boulders and vegetation will create a
large amount of friction, slowing the water down
as it passes over the bed.
In many areas of the world the channel network is
highly variable in time and space. Small channels
may be ephemeral and in arid regions will frequently
only flow during flood events. The resistance to flow
under these circumstances is complicated by the
infiltration that will be occurring at the water front
and bed surface. The first flush of water will infiltrate
at a much higher rate as it fills the available pore
space in the soil/rock at the bed surface. This will
remove water from the stream and also slow the
water front down as it creates a greater friction
surface. Under a continual flow regime the infiltra-
tion from the stream to ground will depend on the
hydraulic gradient and the infiltration capacity.
Baseflow
In sharp contrast to the storm runoff debate, there
is general consensus that the major source of
baseflow is groundwater - and to a lesser extent
throughflow. This is water that has infiltrated the
soil surface and moved towards the saturated zone.
Once in the saturated zone it moves downslope,
often towards a stream. A stream or lake is often
thought to occur where the regional water table
intersects the surface, although this may not always
MEASURING STREAMFLOW
The techniques and research into the measurement
of streamflow are referred to as
hydrometry
.
Streamflow measurement can be subdivided into
