Irrigation, artificial watering of land to sustain plant growth. Irrigation
is practised in all parts of the world where rainfall does not provide
enough ground moisture. In dry areas irrigation must be maintained from
the time a crop is planted. In areas of irregular rainfall, irrigation
is used during dry spells to ensure harvests and to increase crop yields.
The procedure has greatly expanded the amount of arable land and the production
of food throughout the world. In 1800 about 8.1 million hectares (20 million
acres) were under irrigation, a figure that rose to 41 million hectares
(99 million acres) in 1900, to 105 million hectares (260 million acres)
in 1950, and to more than 222 million hectares (550 million acres) today.
Irrigated land represents about 15 per cent of all land under cultivation
but often produces over twice the yield of non-irrigated fields. Irrigation
can, however, waterlog soil, or increase a soil's salinity (salt level)
to the point where crops are damaged or destroyed. This problem is now
jeopardizing about one-third of the world's irrigated land.
The four main methods used today to irrigate fields are flood,furrow,sprinkler,
and drip, or trickle, irrigation.
TROUBLE OF IRRIGATION:
Flood irrigation is used for close-grown crops, such as rice, where
fields are level and water is abundant. A sheet of water is allowed to
advance from ditches and remain on a field for a given period, depending
on the crop, the porosity of the soil, and its drainage. Basin flooding
is used in orchards, with basins built around trees and filled with water.
Furrow irrigation is employed with crops grown in rows such as cotton
and vegetables. Parallel furrows, called corrugations, are used to spread
water over fields that are too irregular to flood.
Sprinkler irrigation uses less water and provides better control.
Each sprinkler, spaced along a pipe, sprays droplets of water in a continuous
circle until the moisture reaches the root level of the crop. Centre-pivot
irrigation uses long lines of sprinklers that move around a circular field
like the large hand of a clock. This method is used especially for feed
crops such as alfalfa, which, when irrigated, furnish several mowings a
Drip, or trickle, irrigation delivers small but frequent amounts
of moisture to the root area of each plant by means of narrow, plastic
tubes. This method, which is used with great success in the United States,
Israel, and Australia, ensures a minimum loss of water through evaporation
or filtration into the ground.
The chief problem caused by continuous irrigation is that of salt accumulating
in the upper layers of the soil and stunting or preventing plant growth.
Nearly all irrigation water, whatever its source, contains some salt, which
filters down to the water table, or level. Where drainage is bad and the
water table approaches root level, the concentrated salt makes plant growth
impossible. Good drainage systems, therefore, which keep the water table
well below the root level and allow water to flush salts through the topsoil,
are now understood to be a crucial aspect of a successful irrigation system.
Increased salinity due to poorly drained soil began to ruin rich lands
of the southern Tigris-Euphrates Valley in Mesopotamia as early as 2100
BC. By 1700 BC yields from these lands dwindled to a quarter of their once
abundant harvests, and the great Sumerian cities that depended on them
went to ruin. Pakistan's huge irrigation project in the Indus Plain has
the same problem, with one-fifth of the land severely affected by 1960.
Measures have been taken to lower the water tables by means of tube wells,
allowing water to flush the salts through the topsoil. Some progress has
Since completion of the Aswân High Dam in the 1960s, the Egyptian
government has had to spend increasing amounts of money to prevent salt
from building up in Nile Valley fields. For previous millennia the Nile
waters removed salt from the land with annual floods, a process that must
now be done by constructing artificial drainage systems.
Although the world's irrigated area grew by 3 per cent a year in the
third quarter of the 20th century, this rate is expected to fall to 1 per
cent in the last quarter, largely because most of the economically feasible
opportunities of large-scale irrigation development have already been exploited.
Also, as greater demands are now being made on limited water resources,
efficient use of available surface and groundwater supplies is becoming
crucial. Irrigation, therefore, which used to be a matter of a farmer watering
the land, is becoming highly technical, calling for the gathering of vast
amounts of information about overall water resources, the quality of the
soil, and the condition of the water table beneath it-a task that often
lies beyond the means of private farming. A major thrust of research today
is to develop techniques to conserve land that is already under irrigation.