Geography Reference
In-Depth Information
from beneath the water table to the ground surface
under the influence of gravity. They are commonly
found on large alluvial fans, where over 100
qanats
may be present (Beaumont, 1971; 1973).
Qanats
can
take many years to construct, yet they often supply
only relatively small volumes of water. This provides
an indication of just how valuable irrigation water
is in such environments. The great advantage of the
qanat
is that once it is constructed, water is delivered
to the ground surface without the need for any
other energy source.
Since the beginning of the twentieth century,
water for irrigation has been stored and supplied
by high-technology methods. With surface waters,
the most important infrastructure feature has been
the major dam and associated reservoir. These
structures have permitted the storage of large
volumes of water, which could then be distributed,
over hundreds of kilometres if necessary through
pipelines and canals to where the irrigation water
is needed. The modern development of
groundwater reserves has been much slower.
Indeed, it really only began after the SecondWorld
War, when reliable mechanised pumps became
widely available. The problem with the use of
pumps is that it is possible to withdraw water from
the aquifer at a rate that is in excess of natural
recharge. Under such conditions, the groundwater
is said to be being 'mined' and the inevitable
consequence is that the water table will fall.
An interesting example of the potential conflict
between modern and traditional methods of
groundwater extraction for irrigation can be seen
on the Varamin Plain of Iran (Beaumont 1968)
(Plate 12.2). The traditional pattern of water use
was to extract groundwater by qanats. The crucial
point here is that the volume of water produced
by the
qanat
depends on the height of the water
table (Figure 12.1). When the water table is high,
the water-producing section of the
qanat
is longer
and hence discharge increases. With a falling water
table the converse is true. Therefore, under
fluctuating climatic conditions
qanat
outputs vary.
During the 1960s, over 100 pumped wells were
sunk on theVaramin Plain to develop new areas of
land for cultivation. The result was that the
regional water table of the plain began to decline,
and as it did so the discharges of many of the
qanats
fell as well. Indeed, a number dried up completely,
causing considerable social dislocation in the
affected villages.
GLOBAL USE OF IRRIGATION
FAO data suggest that the total irrigated area in
the world has grown from 167 million hectares in
1970 to 255 million hectares in 1995, representing
a 53 per cent increase. With an average water use
of around 10,000 m
3
per hectare, this irrigated area
increase implies an extra water usage of 880,000
million m
3
, or the equivalent of ten times the flow
of the River Nile. Currently, the overall pattern of
growth appears to be a steady one, and there is no
indication that the observed rate of increase is
slowing down (Figure 12.2).
The distribution of irrigated agriculture reveals
marked variations from one country to another at
the present day. The countries with the largest
irrigated areas by a considerable margin are India
and China, which both possess around 50 million
hectares of irrigated land (Table 12.1). These two
countries alone account for around 39 per cent of
the world's irrigated lands. Pakistan is next with
17 million hectares, followed a long way behind
by Iran, Mexico, the Russian Federation and
Thailand. In total, twenty-three countries have
irrigated areas of more than 2 million hectares, and
these countries together account for
approximately 75 per cent of all the irrigated land
in the world.
The relative importance of irrigated land
within an individual country depends largely
upon climatic conditions. The drier the country
the more dependent it tends to be upon irrigation.
FAO statistics indicate that in 1995 four countries
—Bahrain, Egypt, Kuwait and Qatar—were
totally dependent on irrigation for any crop
growth (Table 12.1). Perhaps surprisingly, only
fourteen countries were dependent upon
irrigation for more than two-thirds of their
cropped lands, and of these only Egypt,
Uzbekistan and Pakistan can be classed as
countries with large irrigated areas. Even if all
