Geography Reference
In-Depth Information
groundwater is served to communities with smaller populations
which makes quality control more difficult (e.g. from a small well
supplying 100 homes). People have obtained groundwater for thou-
sands of years through digging wells or collecting water from springs
which are zones of groundwater emergence. However, if the
amount of water being withdrawn from the ground by humans is
not replaced at the same rate from inputs to the aquifer then
groundwater levels will fall. Indeed a rise and fall in water table nat-
urally happens seasonally in many areas depending on rainfall and
evapotranspiration
rates. However, in many places groundwater
has been greatly depleted, particularly where rates of replenishment
are slow and the water that is being abstracted may be hundreds or
even thousands of years old. The costs of pumping out deeper water
are greater than water near the surface. Subsidence has resulted in
some places due to groundwater depletion, such as in Mexico City,
or Tucson, Arizona, USA (see Box 4.1). Because of the links
between groundwater and baseflow, rivers can dry up if ground-
water is over-abstracted. In coastal areas, where the aquifer is
connected directly to the sea, groundwater abstraction can lead to the
intrusion of salty water to replace the lost groundwater under land.
This is a problem, for example, along lots of Australian coastlines
where there are major cities, and on the shores of the Persian Gulf.
River flow
Water flowing through and across a land surface is called runoff
(note that runoff does not just mean overland flow). Runoff often
flows into rivers or lakes. The area of land that could potentially
drain into a river or lake is known as the catchment area or
water-
shed
. Rivers can be fed by throughflow or by overland flow and
the relative proportions of the different types of flow can determine
how quickly the river flow changes during rainfall events or sea-
sonally. River flow is crucial for aquatic life, water availability for
reservoirs, abstraction for human use and flooding.
River flows can change during individual rainfall or melt events,
or remain fairly stable, depending on the nature of the environ-
ment. There tends to be a lag between the precipitation occurring
and the peak discharge of a river. This lag time is affected by the
runoff processes discussed above. Where iniltration-excess over-
