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Fresh water supplies are going to run out, so what can we do to make the taps keep running?

This may seem like a surprising statement, but the world’s supply of fresh water is finite. As global population rises, the demand for food — and the water that produces it — grows inexorably. Globally, farming accounts for 70 per cent of our withdrawals from this fixed “bank account”, this in the face of ever-greater domestic and industrial usage.

Water tables are falling in many parts of the world. Himalayan glaciers will shrink massively in the next century, reducing natural water storage in the mountains. The shortfalls will have to come from groundwater and surface storage. Many great rivers have drastically diminished flows.

Bangladesh is suffering from the diversion of Ganges River water and increased salinisation. Underground aquifers in many places are shrinking so rapidly that NASA satellites are detecting changes in the Earth’s gravity. The Water Resources Group has estimated that India may face a 50 per cent lag in water availability relative to demand by 2030 and that global availability may lag demand by as much as 40 per cent; the statistics have been questioned. Sixty years ago, the world’s population was about 1.25 billion people; few people, even in arid lands, worried about water supplies. Then came the Green Revolution, with its new, high-yielding crops, which depend on fertilisers and a great deal more irrigated farming. Global populations skyrocketed to nearly seven billion by 2009, with a projected nine billion by 2050. By the same year, the five hundred million people living in areas chronically short of water in the year 2000 will have grown by 45 per cent to four billion. A billion of us currently go hungry because there is not enough water to grow food. Much of the world’s water is still unpriced, but it is now the most valuable commodity in the world. To compound the problem, 60 per cent of the world’s people live in crowded river basins shared by several countries, often with daggers drawn.

The problems are acute, especially in arid areas with growing populations, where boreholes and aquifers are thought to be the answer. Seemingly a miraculous solution, but not if the drawdown exceeds the replenishment rate, as is the case with the ground water beneath a now-sinking Mexico City’s 20 million inhabitants and with Bangkok, Buenos Aires and Jakarta, where pollution and rising salt levels combine with overdrafting.