Geoscience Reference
In-Depth Information
26.5 Irrigation, dams, reservoirs and disease
Irrigation has been practised in south-west Asia for at least 7,000 years. The world's
oldest recorded dam was built south of Memphis in Egypt under the aegis of Pharaoh
Menes for water supply and irrigation. A ceremonial stone mace-head shows an
Egyptian ruler excavating a canal sometime between 5,400 and 5,000 years ago (see
Butzer,
1976
, p. 21, fig. 2). Basin irrigation of wheat and barley was underway in the
Nile Valley by about 6,500 years ago, and irrigated farming was practised in south-
west Asia at least 1,000 years before then. The primary reasons behind the adoption
of irrigated farming remain the same today as they were 5,000 years ago, namely, to
cope with the vicissitudes of sparse and erratic rainfall and to capitalise on regular
floods from rivers flowing through desert lands. However, what has changed today is
the scale of dam building and the alternative uses to which the dammed water may
be put, notably, to generate hydroelectric power. There are certain long-understood
environmental costs associated with the construction of dams and reservoirs in the
drier regions of the world (Kassas,
1972
), of which the spread of water-borne disease
is perhaps the most serious. The spread of Schistosomiasis (or Bilharzia) as a result
of the reduction in river-flow velocity and the creation of artificial lakes has been
a scourge in many parts of Asia, Africa and South America. The disease is caused
by infestation with parasitic blood flukes transmitted by aquatic snails (Williams and
Balling,
1996
, pp. 137-140). An infected human can harbour about 4,000 pairs of
flukes/worms, leading to destruction of the affected tissues. The parasites do not
kill their human host but do cause weakness and anaemia from blood loss. Various
chemical methods have been used to kill the snail carriers, or vectors, in the water,
none of which are cheap, and some methods of biological control appear promising, in
that they do not kill fish (Lemma,
1973
). Other diseases associated with the presence
of large bodies of standing water are malaria (the mosquito larvae live at the water
surface) and filariasis, which is transmitted by a species of fly that lays its eggs in still
water. All too often in their enthusiasm for dam building, government authorities fail
to factor in the costs of disease and to make disease prevention an essential part of
their planning.
26.6 Keeping fossil groundwater use in balance with aquifer recharge
We have seen in earlier chapters that the contrast between the late Pleistocene (LGM)
aridity of the Sahara, the Gobi and the Thar deserts and their well-vegetated and
well-watered early Holocene condition had a considerable impact on the Late Stone
Age/Upper Palaeolithic and earlyNeolithic peoples who benefited from these changes.
As the great continental ice sheets melted, postglacial temperatures and sea levels
rose around the world. Evaporation from the intertropical oceans increased once sea
surface temperatures became warmer. The summer monsoons of tropical Australia,
