978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

Transfers
Overland flow is the movement of water over the land, downslope to a body
of water. It has two main mechanisms. Where precipitation exceeds the
infiltration capacity accumulated water will flow downslope due to the effects
of gravity. An alternative mechanism occurs when the soil saturation exceeds
its maximum capacity due to groundwater uplifting, base flow, and lateral
subsurface water discharges, resulting in the appearance of saturation excess
overland flow.
Channel flow is the movement of water within a defined channel such
as a stream or river. The speed and flow of the water will depend on a variety
of factors such as gradient and efficiency; these are considered in more
detail in river channel processes and landforms (pages 20–29). Base flow is
considered to be the lowest flow within a channel, often occurring due to a lack
of precipitation leaving only the influence of water trapped in rocks and soil.
It is maintained by groundwater seeping into the bed of a river. The channel
is topped up by precipitation events and the arrival of water through other
mechanisms such as throughflow, overland flow etc. It is relatively constant but
increases following wet conditions.
Throughflow refers to the movement of water through the soil substrata.
As the soil type of an area is closely linked to the underlying bedrock flow
rates through different soil profiles can be varied. Clay-rich soils are known for
their water retention whereas sandy loams are characteristically free draining.
The influence of land use also plays a part as it can influence soil density and
aeration (page 19).
Groundwater flow is subsurface water (lies under the surface of the ground)
that travels downwards from the soil and into the bedrock through cracks and
pores. This process is called percolation.
Differing rock types and structures will affect the flow of water into
underlying layers, with porous sedimentary/carboniferous rocks such as chalk
and limestone being the most effective carriers of water. The layers of rock that
become saturated form the phreatic zone (Figure 1.7 (a)) in which the uppermost
layer is known as the water table. Where there is a small area of underlying
impermeable substrata (aquiclude), water may be held higher up the basin
profile as a perched water table (Figure 1.7 (b)). Water that cannot pass through
the rock layers will emerge as a spring.
Outputs
Evaporation is the process by which water is converted to water vapour
in the atmosphere. This is most significant where there are large bodies
of water such as the oceans and seas and on a local scale – rivers
and lakes. Rates of evaporation are dependent on climatic variables
such as temperature, humidity and wind speed. Other factors include the
spring
perched water table
aquiclude
unsaturated zone
river
(dry in summer)
zone of intermittent saturation
winter water table
summer water table
water table
river
aquifer
figure 1.7 (a) Seasonal variation in the level of the water table.
figure 1.7 (b) Perched water table
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Hydrology and fluvial geomorphology