Geography Reference
In-Depth Information
infrastructure costs associated with them are high
(Plusquellec
et al
. 1994). The commonest of these
methods is the sprinkler system, which delivers
water to the crop from overhead in droplets that
are similar in form to rainfall (Plate 12.1). These
can vary in size from small individual 'gun-like'
structures that have to be moved by hand around
a field to self-propelled centre-pivot systems with
overhead arms up to 500 m or more in length.
Other widely used methods are trickle and drip
systems, which are employed for highervalue
crops. Flexible or rigid pipes are laid along crop
rows and the water trickles or drips from holes in
the pipes into the soil adjacent to the crops. In
some cases, the pipes can actually be buried into
the soil so that the water is delivered directly to
the root zone of the growing crop, With trickle
and drip systems, the volumes of water used can
be strictly controlled, so water wastage rates are
very low. Fertilisers and other chemicals can also
be added directly to the water before it is delivered
to the crops.
Poorly designed or badly operated irrigation
projects can cause serious environmental problems
(ESCAP 1989; Gelburd 1985). The main cause of
problems is usually the application of excessive
amounts of water. The water that enters the soil is
quickly evaporated from the ground surface,
leaving behind salts in the upper layers of the soil,
which reduce crop growth or may even
completely prevent it (Misak
et al.
1997). Once
soils are affected by salinisation, it is extremely
difficult for them to be reclaimed, and costs are
always high. Soviet Central Asia is a region that
has been particularly badly affected by poor
irrigation practices, with the result that thousands
of hectares of land have had to be abandoned
(O'Hara, 1997). Similar problems have occurred
in the Murray-Darling basin of southeast Australia,
although here the main causes have been the
removal of the natural vegetation and the high
salinity levels of local soils and waters (Mackay and
Landsberg 1992; Simpson and Herczeg 1991).
For irrigation to be possible, a reliable source
of water is essential. This can be obtained from
either surface or groundwater sources. The very
large volumes of water that are necessary for
irrigation, often in excess of 10,000 m
3
per hectare,
have meant that in traditional societies
groundwater from wells has not been a significant
means of obtaining water except for small gardens
(Beaumont 1993). Water is extremely heavy, so its
extraction from the ground is highly energy-
intensive.
In many of the drier parts of the Middle East
and Central Asia, the extraction of groundwater for
irrigation has been achieved by the use of
qanats
.
These are underground tunnels that lead water
Plate 12.1
Modern centre
pivot irrigation system,
southern Spain.
