104 / Basics of Environmental Science
cent of all farmland is irrigated, ranging from 6 per cent in Africa and South America to 31 per cent
in Asia. Between 1970 and 1990 this area increased by more than a third, from 168 million ha to 228
million ha, most of the increase being in developing countries, and the output from irrigated land is
more than double that from unirrigated land; one-third of the world’s food is grown on irrigated land
(TOLBA AND EL-KHOLY, 1992, p. 290).
Water for irrigation is often provided by damming rivers to fill reservoirs, the flow of water from the
dams also generating electrical power, but large dams can produce adverse environmental effects.
Their reservoirs flood large areas, destroying existing plant and animal communities and often
displacing many people, and silt carried from upstream tends to accumulate, gradually filling the
reservoir. Where rivers formerly flooded land downstream at a certain time of the year, the silt
deposit containing plant nutrients is lost to farmers, who must buy fertilizer to replace it. In seismically
active regions, large dams may also be linked to increases in the number of earthquakes. An earthquake
exceeding magnitude 5 on the Richter scale occurred while the first large dam in the world, the
Hoover Dam on the Colorado River, was being filled in 1936 and there was another of comparable
magnitude in 1939. There have also been earthquakes greater than magnitude 5 associated with the
Koyna Dam, India (1967), Kremasta Dam, Greece (1966), Hsinfengkiang Dam, China (1962), and
Marathon Dam, Greece (1938), each of them accompanied by foreshocks and aftershocks (GOUDIE,
1986, pp. 243–244).
Land can be irrigated simply by flooding it and allowing the water to sink into the ground. A somewhat
more sophisticated method is to dig parallel furrows down the slope of a field and fill them with
water from a ditch or pipe across the upper edge of the field. A more familiar technique involves the
use of sprinklers. These are versatile, in that they can be moved to where they are most needed and
the amount of water they deliver can be controlled closely. In some places, irrigation is supplied by
subsurface pipes.
Environmentalists used to be fond of saying ‘everything has to go somewhere’. This is as true of
water as of anything else and water supply is only one side of the water management equation: water
must also be removed. In some places, wet ground can be rendered cultivable only by making it
drier; in others, irrigation must be accompanied by improved drainage.
Land drainage is a farming practice probably as ancient as irrigation. On sloping ground, a ditch
along the upper boundary of a field, at right angles to the direction of slope, will collect water
draining from higher land before it flows into the field. A network of communicating ditches can
then carry the surplus water to the nearest stream.
On level ground, or where the construction of ditches is insufficient, drains may be laid below
ground. The simplest technique is to install ‘mole’ drains, so called because the implement that
makes them tunnels through the soil like a mole. The ‘mole’ itself is a metal cylinder fixed to
the lower end of a bar, buried to the desired depth, and then towed through the soil. Figure 3.7
illustrates the device and shows that it makes a hole parallel to the surface. In most soils the
hole will remain open for some years before the operation needs repeating. More permanent
drains are made from short lengths of perforated piping laid end to end by a machine that digs
the trench into which it lays them, then buries them as it passes. In both cases the drains feed
into a stream or system of ditches. The land area drained is proportional to the depth of the
drain, so it is a simple matter to plan a drainage system that will serve a whole field without
leaving wet patches.
It is easy to see why farmers find it desirable to remove surplus water from wet ground. The need for
a drainage system to accompany irrigation is less self-evident, but lack of drainage on irrigated land
is a major cause of soil degradation.