Regular readers of New Scientist know that Fred Pearce is the scourge of dam builders, especially those with near-megalomania about vast barriers and reservoirs.  Back in the late 1960s Canadian environmentalists were horrified to learn of plans being developed to divert southwards water that naturally flows along the great rivers of the Canadian Shield to the Arctic Ocean and Hudson’s Bay.  This was NAWAPA, the North American Water and Power Alliance.  NAWAPA is still a live ambition for supplying the water-hungry west and mid-west states of the USA.  The former Soviet Union put such grandiose plans into effect, one outcome being the dramatic shrinkage of the inland Aral Sea.  Pearce returns to continental water transfer in an important review in the weekly for whom he has worked for many years (Pearce, F.  2003.  Replumbing the planet.  New Scientist, 7 June 2003, p. 30-34).  His trigger is the filling of the giant Three Gorges reservoir on the Yangste, one of whose aims is to channel water northwards to augment supplies to the increasing parched plains of central eastern China.  But this is only the start of an awesome venture, that will also shift the equivalent of 25% of the Nile’s flow from Tibet’s glacial meltwater that feeds the Yangste into the Yellow River, which now barely trickles into the Yellow Sea.  India seems bent on snaffling much of the flow from the Ganges and Brahmaputra catchments into the drought-prone south of the subcontinent.  As well as the huge disruption of people and environment that schemes such as these must entail, Pearce highlights the vast economic costs.  India’s continental engineering will eat up the equivalent of 40% of its GNP. 

Obviously, such huge ventures throw up equally large political and ethical questions, which are not easy to resolve.  In many cases the perceived needs for regional water transfers stem from very wasteful water use, particularly in agriculture.  Using drip or trickle irrigation, which needs large-scale application but relatively low-cost and simple technology can reduce water requirements dramatically, simply by reducing losses by evaporation from canals.  In semi-arid areas as much as 70 % of channelled water never reaches the crops for which it is intended.  Governments such as those of India and China depend so much on rural support that they might commit political suicide by pressing for changes to practices that date back millennia, so they opt for the spectacular, quick fixes.  Yet there are other such schemes that might transform the livelihoods of some of the worlds most destitute people in the Sahel and Horn of Africa.  One suggestion is to divert part of the largely unused river flow through humid tropical Central Africa across the Sahel to reach Lake Chad.  Another, not mentioned by Pearce, is to dig a channel that will flood the Danakil Depression of Ethiopia and Eritrea, which lies about 100 m below sea level.  Topographically, this would be relatively easy, because only about 30 km of low-lying coastal plain separates the Red Sea from the Depression.  The flow could generate hydropower in a power-starved region, and evaporation from the resulting saline lake would boost rainfall in the world’s hottest place, and perhaps allow harvesting of the many salts that would be precipitated, including potash fertilisers.  Solar energy could also be used for low-cost desalination.  However, no-one can guess at the climatic and ecological consequences of changing humidity in both the Chad and Danakil basins.   Yet, water is becoming the most strategically important physical resource so rapidly that the enormous economic implications for transnational contractors, and political prestige associated with regional transfer schemes will drive them ever onwards.  There is one glimmer of hope, which Pearce mentions; ordinary people in Rajasthan, India’s driest state, have resurrected old practices of water harvesting, and find that they are more secure than those who rely on state-sponsored canal supplies.  The root issue is that rainfall disappears either by run-off or evaporation in a matter of days, unless it is stored somehow.  Any habitable place has rainfall, albeit irregular in drought-prone areas, and quite low-cost ingenuity can “bank” the transient spates where the water is needed.