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

Precipitation
In heavily industrialised areas and urban spaces precipitation rates are as much
as 10 per cent higher due to an increased number of pollutants and particulate
matter creating a greater extent and frequency of clouds.
For moisture to fall as rain, water vapour must attach to small particulate matter
in the atmosphere known as hygroscopic nuclei. As water vapour accumulates and
condenses to form clouds, droplets of water increase in size before falling under
the influence of gravity. According to Colorado’s National Centre for Atmospheric
Research (NCAR) there are over 150 legitimate weather modification programmes
taking place in 37 countries, though their complexity and cost vary greatly.
Cloud seeding is one strategy designed to encourage precipitation. Cloud
seeding injects more particulate matter into the atmosphere in order to create
rain. Silver iodine, carbon dioxide and ammonium nitrate are used and dispersed
either by aircraft or more commonly fired by cannon or rocket into the air.
The result of cloud seeding is largely inconclusive. In Australia it has been
suggested that precipitation has increased by 10–30 per cent on a small scale
and short-term basis. China is investing heavily in the technology with the
introduction of 40 000 field operatives.
Land use change
Urbanisation
An increase in urbanisation creates large impermeable surfaces, which reduce
the amount of interception and infiltration.
Urbanisation has a close relationship with flashy hydrographs. As water runs
over impenetrable surfaces and into drains it is carried rapidly resulting in a
quicker response in the river, raising levels and increasing flood risk. An increase
in urban surfaces increases runoff and the potential for flooding.
Deforestation and afforestation
The effect of vegetation removal on hydrology and streams, through land clearance,
is a common theme on populated landscapes. Now less than 1 per cent of Britain
is covered by natural woodland due to the expansive activities of humans. Whether
for land clearance, development or crop harvesting, the removal of vegetation can
have profound effects on the hydrological balance of an area. Where clearance is
large in relation to the vegetative coverage the effects will be heightened.
The rates of interception are determined by the type and extent of vegetative
cover. Much of the land’s surface has experienced some level of clearance and
modification, resulting in widespread deforestation. Deforestation reduces
evapotransipiration rates and increases surface runoff, resulting in a flashier
response and shorter lag time. Afforested areas will have a greater capacity to
absorb moisture and help bind the soil. Afforested areas are largely planted for
figure 1.32 Forest removal, Derbyshire, UK
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Hydrology and fluvial geomorphology