Geography Reference
In-Depth Information
lead to rapid and large flood peaks. In some soils such as peat only a
small amount of infiltration may be needed to cause the water table
to rise to the surface. In other soils there may even be two river dis-
charge peaks caused by one rainfall event. This might occur where
the first peak is saturation-excess overland flow dominated and the
second peak a little while later may be much longer and larger and
caused by subsurface throughflow accumulating at the bottom of
hillslopes and valley bottoms before entering the stream channel.
Throughflow may also contribute directly to storm hydrographs by
a mechanism called piston or displacement flow. This is where soil
water at the bottom of a slope is rapidly pushed out of the soil by
new fresh infiltrating water entering at the top of a slope.
Figure 4.1 shows flows in the form of hydrographs over one
year for two nearby rivers where the climate is the same. Despite
being in the same area the flows are very different between the
rivers. The flows in the River Nol appear to be dominated by
baseflow and there are no individual storm peaks unlike for the
River Creef which appears to be more dominated by overland
flow or rapid throughflow (e.g. via macropores). The River Nol
catchment overlies permeable bedrock, is gently sloping, and has
soils that enable good infiltration and little chance of saturation to
the surface. Therefore, iniltration-excess or saturation-excess over-
land flows are a rare occurrence here. For the River Creef,
however, the soils are thin and sit above impermeable bedrock and
so there is frequent saturation of the soils and generation of
saturation-excess overland flow.
The proportion of rainfall that reaches a river channel may vary
from close to 100 per cent to 0 per cent. Seasonal variability in
river flow is known as the
river regime
for which there are four
major global types. Where snow and ice melt dominate then there
can be a major peak of river flow during the late spring in the case
of snow melt or early summer with annual glacier melt. River dis-
charge (the volume of water passing a point in the river per unit of
time) can be extremely low during the winter months even though
precipitation may be continuing as this precipitation is stored on
the glacier in winter. There can also be a strong daily change in
river discharge due to daily melt cycles of the snow and ice.
Night-time discharge tends to be much lower than that of the
mid-afternoon.
