Geography Reference
In-Depth Information
12
Irrigation
Peter Beaumont
INTRODUCTION
Irrigation is the addition of water to the soil to
produce near optimum soil moisture conditions
for crop growth in regions of water scarcity
(Rydzewski and Ward 1989). Irrigated agriculture
appears to have commenced on a small scale soon
after the domestication of cereals in the Middle
East. This domestication probably began about
12,000 years ago, yet by 10,000 years ago
archaeological excavations at Jericho have revealed
that groundwater from a nearby spring was being
used for irrigation (Kenyon 1969-70). However,
irrigated agriculture is perhaps most closely
associated with the development of urban
civilisations in the great river valleys of the Old
World, including the Tigris-Euphrates, Indus and
Nile (Adams 1965; Wittfogel 1957). The key factor
is that irrigation of these huge floodplains using
large-scale diversion structures and canals
generated the wealth necessary for the
construction of complex urban systems and the
founding of empires.
Irrigation, by providing more assured crop
production and by increasing yields by up to four
times those of their rain-fed equivalents, meant
that much larger populations could be supported
from relatively small areas of cultivated land. In
turn, this creation of wealth from agriculture led
to the division of labour within society and the
growth of specialist occupations, such as soldiers,
enquirers and accountants, which are taken for
granted today. Irrigation development can,
therefore, be regarded as a vital element in the
evolution of human societies.
THE NATURE OF IRRIGATION—
METHODS AND PRACTICES
There is a continuum of irrigation practices in
terms of the volume of water added, from crops
such as wheat, which require relatively little water,
even in arid environments, to crops like rice,
which can demand very large quantities of water
even in humid environments (Burns 1993). The
earliest methods of irrigation were probably
furrow and flood types. Both employ low
technology and require only small amounts of
human or animal labour to carry them out. With
furrows, a series of v-shaped channels are
fashioned across the irrigated area using a form of
plough-like implement. The furrows slope in a
downslope direction to ensure that the water
moves along them under the influence of gravity.
With flood irrigation, small banks are constructed
to delimit roughly rectangular flat areas of various
sizes into which water can be led to flood the
ground to a depth of several centimetres. Water is
then usually led from one small basin to another
in a downslope direction. Basin or flood irrigation
is best known from the Nile valley (Hamdan
1961).
Since the end of the Second World War, new
methods of irrigation have been developed. They
are usually described as pressure systems, as the
water is delivered to the plant/crop through a
series of pipelines. All these systems require a much
higher level of technology than is the case with
the traditional furrow and flood systems, and the
