Irrigation - from the Latin rigo, meaning to moisten - was practised by the Egyptians 6,000 years ago when they dug channels from the Nile to grow crops on the arid surrounding land. As cities grew, water was needed for sanitation. In 2500BC the 35,000 inhabitants of Mohenjo Daro on the banks of the Indus in Pakistan enjoyed sophisticated bathing and drainage facilities, and by 400BC the Romans had built aqueducts, sewers and drains to service their capital city.
Irrigation enables farming where rainfall is insufficient or unpredictable. A quarter of the agricultural land in Greece and Italy is irrigated, compared with just 2 per cent in the UK. Large tracts of eastern England were under water until the fens were drained in the 17th century and, though much of this land is below sea level, embankments keep it above water.
Britain's newest waterway - the 12km Jubilee river near Maidenhead - is in fact a naturalistic-looking flood relief channel.
But this kind of liquid engineering has not always run so smoothly. Ever since 1894 when Thirlmere in the Lake District was flooded to slake Manchester's thirst, the dambuilders have met environmental opposition. The proposed Ilisu dam in Turkey will bury Kurdish homelands and ancient monuments while the biggest dam in the world, the Three Gorges on the Yangtse river, due to be completed in 2009, will displace 1.9 million people.
Tampering with nature's waterways can have catastrophic effects. The Aral sea between Kazakhstan and Uzbekistan was once the fourth largest lake in the world. Its shores have receded by more than 50 miles in the past 30 years, leaving fishing towns high and dry and what remains of the water seriously polluted. Most of the water taken for irrigating cotton fields has been wasted, evaporating or leaking on its way to plantations. We live in a world of apparent plenty, where water covers two-thirds of its surface. But it is, in fact, a finite resource and only 1 per cent of it is fresh water accessible to humans. Where should it go and who should get it? For governments in water-poor countries the big environmental debate today is irrigation, irrigation, irrigation.
Watery places to visitPinchbeck Land Drainage museum, Spalding, Lincolnshire. Tel: 01775 725468 Jubilee river, Berkshire - the Thames takes a new shortcut between Maidenhead and WindsorLadybower dam, Derbyshire - Britain's biggest dam when it opened in 1945