Earth: The parched planet

But for all its virtues, Chowdhury's 2-hectare farm is sowing the seeds of a global disaster. To grow the fodder that he needs to feed his cows, he is entirely dependent on irrigation water pumped from deep underground. Over the course of a year, his small electric pump sucks twice as much water from beneath his fields as falls on the land as rain. No wonder the water table in the village is 150 metres down and falling by 6 metres a year.

What is less well known is that the success of this "green revolution" was built on a massive investment in irrigation systems. Today the world grows twice as much food as it did a generation ago, but it uses three times as much water to grow it. Two-thirds of all the water abstracted from the environment goes to irrigate crops. This use of water is massively unsustainable, and has led many people to conclude that the apocalypse wasn't averted, only postponed.

The starkest example is India. Over the past decade, the country has seen an extraordinary "barefoot" hydrological revolution. Farmers have hired drilling rigs and bought electric pumps to mine water that has sat undisturbed beneath their fields for millennia. Today, more than 21 million Indian farmers tap underground reserves to water their fields, and two-thirds of India's crops are irrigated with underground water.

The juggernaut is still accelerating. There are a million more pumps every year. We are only just beginning to see the consequences." Shah estimates that at least a quarter of Indian farmers are mining underground water that nature will not replace, and that up to 200 million people face a waterless, foodless future.

The groundwater boom is turning to bust and, for some, the green revolution is over. Fifty years ago in northern Gujarat, bullocks driving leather buckets lifted water from open wells dug to about 10 metres. Now tube wells are sunk to 400 metres, and they still run dry. Half the traditional hand-dug wells and millions of tube wells have dried up across western India. In the southern state of Tamil Nadu, two-thirds of the hand-dug wells have failed already, and only half as much land is irrigated as a decade ago. Whole districts in Tamil Nadu and Gujarat are emptying of people. Suicides among farmers are rife. Many more are joining the millions migrating to urban slums or joining the gangs of construction workers and labourers travelling the roads of India.

His electric pump brings up 12 cubic metres of water an hour. When he needs to irrigate his fields, which he does 24 times a year, it takes 64 hours to pump up all the water he needs. That adds up to 18,000 cubic metres of water a year to grow the fodder to produce just over 9000 litres of milk. That's 2000 litres of water for every litre of milk. According to Shah that is better than the local average.

In the backwoods of Gujarat, I met Haradevsinh Hadeja, a retired Indian police officer who has transformed his home village of Rajsamadhiya by doing just that. He has turned a near-desert landscape of desiccated fields and empty wells into a verdant scene of trees, ponds, full wells and abundant crops. Most of the other villages in the area rely on government water tankers to provide drinking water for much of the year. They have little left to irrigate their crops. That's not the case in Rajsamadhiya. "We haven't had a water tanker come to the village for more than 10 years. We don't need them," Hadeja says.