Geography Reference
In-Depth Information
chemical properties) and changes to the chemistry of the water.
However, some soils have lots of macropores within them, many
of which actively transport water. At some sites measurements have
suggested half of the water moving through soils could move
through macropores. If an arable field has many macropores,
surface fertiliser applications may get washed through the macro-
pore channels and may not enter the main part of the soil. In this
situation, the fertiliser will not be available for crop use and may
enter the stream system leading to water quality problems. Macro-
pores can be formed by soil animals, plant roots and cracking
during dry conditions or from small landslips. Some macropores
can be several metres in diameter, particularly in erodible soils such
as in semi-arid south-east Spain or Arizona or in karst limestone
cave systems. Turbulent flow within these large macropores in soils
can rapidly enlarge them further until eventually they collapse to
form gullies.
Groundwater
The term groundwater can be defined in many different ways.
Some view it solely as water within rock. Another working defini-
tion is that groundwater is water held below the water table both
in soils and rock. In any case water held within the ground is of
worldwide importance. In many catchments water is supplied to
the stream from groundwater in the bedrock. This is water that has
percolated down through the overlying soil and entered the
bedrock. Rock has small pores, fractures and fissures. Groundwater
stores around 30 per cent of the Earth's freshwater (i.e. not salty).
However, if this groundwater is to be available to supply river flow
the rock or soil needs to be permeable, enabling water to flow
through it. Layers of rock porous enough to store water and per-
meable enough to allow water to flow through them in economic
quantities are called
aquifers
.
Some countries, such as Austria, Hungary and Denmark, almost
entirely rely on groundwater. France obtains 56 per cent of its
drinking water from groundwater while the UK obtains around 30
per cent from this source. The USA obtains around half of its water
for all uses from groundwater with a quarter of drinking water
coming from aquifer resources. However, a greater proportion of
