Geoscience Reference
In-Depth Information
Sadly, many irrigation schemes are not well managed and a number of environmental prob-
lems are frequently experienced as a result, both on-site and off-site. In many large net-
works of irrigation canals, less than half of the water diverted from a river or reservoir
actually benefits crops. A lot of water seeps away through unlined canals or evaporates be-
fore reaching the fields. Some also runs off the fields or infiltrates through the soil, unused
by plants, because farmers apply too much water or at the wrong time. Much of this water
seeps back into nearby streams or joins underground aquifers, so can be used again, but
the quality of water may deteriorate if it picks up salts, fertilizers, or pesticides. Excessive
applications of irrigation water often result in rising water tables beneath fields, causing
salinization and waterlogging. These processes reduce crop yields on irrigation schemes all
over the world.
Many of these difficulties have plagued farmers in the Central Asian states of Turkmenistan
and Uzbekistan where desert conditions mean that more than 90% of agriculture relies on
irrigation from the Amu Darya and Syr Darya rivers. A rapid expansion of irrigation in
Central Asia was initiated in the 1950s during the Soviet era, with some dramatic conse-
quences. By the 1980s, the irrigated area had more than doubled to occupy about 7 million
hectares. As a result, the annual inflow to the Aral Sea from the two rivers, the source of
90% of its water, had declined by an order of magnitude from about 55 cubic kilometres a
year to some 5 cubic kilometres annually.
Unsurprisingly, the Aral Sea has become considerably smaller in consequence. In 1960, it
was the fourth largest lake in the world, but since that time its surface area has more than
halved, it has lost two-thirds of its volume, and its water level has dropped by more than
25 metres. In some areas, the Aral Sea's remaining waters are more than twice as salty as
sea water in the open ocean. Most of the lake's native fish and other aquatic species have
disappeared, unable to survive in the salt water, meaning an end to a once-major commer-
cial fishing industry. Receding sea levels have also had local effects on climate, and the
exposed sea bed has become a source of major dust storms that billow out over surrounding
agricultural land up to several hundred kilometres from the Aral's coastline. This fine dust
is laden with salts, adding to the problems of irrigated agriculture. It is also thought to have
damaging impacts on human health.
Effects on fish
People have directly affected the biology of rivers over a very long period. In Europe, the
common carp is found in the rivers of every country, but the fish is native only to the
