Chennai is the largest city in Tamil Nadu, one of the country’s most important agricultural regions. Common sense and a rural tradition of coöperation would suggest that farmers who live near the city share their water and use it sparingly. The agricultural policies of India’s government insure the opposite. Rice is the most popular grain in the world, but it requires far more water than any other crop—typically twice as much as wheat and up to ten times more than lettuce. Yet rice and wheat are the two crops that the Indian government supports through price guarantees, so farmers have little incentive to grow anything else or to use less water. On the same amount of land that Chinese farmers grow four thousand kilograms of rice each year, Indians grow no more than sixteen hundred, and they use ten times more water to do it than is necessary. Near Chennai, rice is all that farmers grow.

I visited Ganasekemon’s village with N. Parasuraman, a water specialist who works at the M. S. Swaminathan Research Foundation, which is dedicated to preserving the region’s environmental resources. Swaminathan is eighty-one and a national hero. In the nineteen-sixties, the country experienced several nearly catastrophic famines. Swaminathan, a plant geneticist, was the head of the Indian Agricultural Research Institute at the time. By combining seeds developed by Norman Borlaug, an American agriculture expert, with local strains of rice and wheat, he helped launch the Green Revolution. The results astonished the world. Yields improved so dramatically that India, which would not have survived without massive imports of grain from the United States, soon became one of the world’s biggest exporters. Swaminathan’s sophisticated hybrids benefitted greatly from the targeted use of pesticides and fertilizer. More than that, though, the Green Revolution was driven by an almost limitless use of water.

Before the nineteen-sixties, groundwater played no real role in farming, and wells were rarely used to irrigate crops. When the amount of rainfall decreased by twenty per cent, so did the grain harvest. By the late eighties, however, this, too, had changed. In 1987, a year in which rainfall was thirty per cent below normal, the production of grains fell by only five per cent. The difference was due to groundwater. “We couldn’t possibly exist without a good well,’’ Ganasekemon said to me. “I don’t know how anyone ever did.’’ Ganasekemon’s use of water is excessive but not unusual: whoever owns land also owns the groundwater beneath it. The water is free, and the electricity needed to pump the water to the surface is extremely cheap. The electrical subsidy for agriculture makes up nearly half of Tamil Nadu’s large deficit.

Everything is for sale in the gray area between urban India and its farmlands. Hawkers offer banana chips, old shoes, and cellular-telephone service. In many parts of the country, the roads are lined with fruit merchants selling fat yellow mangoes or pyramids of limes. Around Chennai, though, water is the ripest fruit. I counted more than a dozen brightly painted twelve-thousand-litre water tankers, each bearing a different company name: Indira Water Supply, Thiramlu Water Supply, Mahindra, Shree Krishna Sharashine, Beven, High Class, Hrahana. As we drove along the dusty roads, Parasuraman, who grew up near Chennai, explained why there were so many tankers: “Indian farmers are good capitalists, and, when a good capitalist has a product that everybody wants, he sells it.’’ These days, water earns more than rice. A local farmer told me, “I have three acres of land, and spend around seven thousand rupees”—about a hundred and fifty dollars—“an acre. My entire family works on the farm for six months of the year, at the end of which I might get twelve thousand rupees per acre. Most of it goes toward paying interest on loans. I have a two-hundred-square-foot well, and it gives me more income than farming does.”

Permitting farmers to exploit the nation’s most valuable resource has led to inequities that are even more striking than those in the cities: rich men plunder their land at will, installing powerful bore wells driven by engines that can draw the water not only from their farms but also from the land of their neighbors—to whom they then sell that water. The day before I went to Vellavedu, I had visited S. Janakarajan in his office at the Madras Institute of Development Studies. Janakarajan, an intense man with bunches of white shooting through his mop of black hair, has written widely on the water conflicts between Indian cities and the rural areas surrounding them. “This is just a mad race,’’ he said. “I call it ‘competitive deepening.’ You deepen your well and suck my water out, so I have to deepen my well even further to get yours. You went down sixty feet, so I will go to seventy feet. This is going on all the time, but it cannot continue indefinitely. There is one chance to get this water. If you win, somebody else loses.’’

He stood up, shook his head, and walked to a map of Tamil Nadu. “If there is an aquifer that should be shared by only four people, it is shared by ten. That way, nobody benefits. But who stops it? There is no law against it. No real property laws. You just have millions of farmers trying to drain the same wells. The entire irrigation system is based on competition, not on sharing. And certainly not on the idea of conservation.” The ponds, lakes, and reservoirs around Chennai have been badly neglected. In many cases, as the city has spread, real-estate developers have simply built over the reservoirs, or used them as toxic dumps. “Chennai does not really have a water crisis,’’ Janakarajan said. “This is a man-made crisis, a policy crisis. Politicians love to talk about architecture and new buildings. Water bores them. They don’t want to plan for growth, so growth makes its own rules.”

Many of the new wells in the area were drilled on the edge of the road, like gas stations, which makes it easier for the trucks to gain access. I stopped in front of the V.B.R. Drinking Water company—a single tanker that sells water to people in Chennai after buying it from local farms. The truck makes a dozen trips each day, and the proprietor, whose name was Selvaraj, assured me that he turns a nice profit. He said that he had been running the business for two years, but he wasn’t eager to elaborate. “We sell what people buy,” he said with a shrug. The G.M.R. Water Supply company was just a hundred metres down the road. A giant hose snaked from the back of a shed and into the tanker. The heat muffled all sound except the furious banging of a pump in the field.

Parasuraman and I drove on, to a farm next to the local Coca-Cola bottling plant. When Indians complain about water, they complain about Coke, which has become a symbol of the intrusiveness of foreign companies. “They come here and take what they want,” one of the farmworkers told me. “As much water as they can get.” It is one of the farmers’ most firmly held convictions, but it happens not to be true. Unlike local farmers, Coca-Cola pays for its electricity at market rates. “The Indian approach is that industry should just pay, pay, pay,’’ John Briscoe told me when I met with him in Washington, D.C. Briscoe, the World Bank’s country director in Brazil, was for many years the bank’s senior water adviser. “Industry uses a small fraction of the water, and it is supposed to pay a hundred per cent of the bill,” he said. “It’s legalized madness.”

Any call for change is greeted by farmers saying that they will die without their human right to water. “If it’s a human right for farmers, shouldn’t it also be a human right for people in the slums, or poor people on the land in villages?” Briscoe said. “I am a hugely optimistic person, and I think that most problems are overblown, they can be corrected and prevented. The problem with groundwater is that it actually can become irreversible. If you wait too long and waste too much, there is no way back. I worry that that is happening in India. They need innovation. More conservation, more variation in crops. They need to harvest their water and charge for its use. And of course they must have more storage. Much more storage. You can’t live on the amount of water they store in India.’’ In the world of hydrology, storage is a code word. It means “dam,” and nobody wants to talk about dams.

The Chinese character for “political order” is based on the symbol for “water,” and the meaning has always been clear: those who control water control people. For centuries, the most effective way to control water has been to build a dam. No public works have had greater impact on their environments than the world’s many colossal dams, and the largest by far were built in the twentieth century. It took sixty-six million tons of concrete to construct the Hoover Dam, which tamed the Colorado River and formed Lake Mead, a reservoir that holds more than nine trillion gallons of water. In Egypt, the Aswan High Dam required twenty times more stone than was used in the Great Pyramid of Giza. When the Three Gorges Dam, on the Yangtze River, is completed, in 2009, it will be the biggest hydroelectric dam in the world. “One of the things Hoover set in motion was a change in the character of the world’s waterways, permanently altering the ecosystems of entire drainage basins,’’ Marq de Villiers wrote in his compelling cultural history, “Water.” “And in at least one case, the Nile, permanently changing a flow pattern that had sustained civilization for five thousand years.”

Few people understood the power of a dam to influence the life of a nation better than Jawaharlal Nehru, India’s first Prime Minister. Dedicating the Bhakra Dam, in 1963, he said, “Bhakra-Nangal Project is something tremendous, something stupendous, something which shakes you up when you see it. Bhakra, the new temple of resurgent India, is the symbol of India’s progress.” Dams, and the large projects that often come with them—pipelines, aqueducts, water-filtration plants—have benefitted billions of people. By the middle of the twentieth century, they had become a defining symbol of man’s attempt to govern nature, an effort that was nowhere more vigorous than in the United States, where there are more than seventy-five thousand dams. “That is a new dam nearly every day since we signed the Declaration of Independence,” Peter Rogers, the Harvard professor, pointed out. “The environmental impact of these things cannot be ignored.” Large swaths of the American West wouldn’t be habitable if not for the dams along the Colorado River. In 1933, poverty in much of the Tennessee Valley was acute, crop yields were low, and there was no electricity. Then President Franklin Delano Roosevelt created the Tennessee Valley Authority. The T.V.A. built forty-two dams and reservoirs, which harnessed enough water to generate electricity for tens of thousands of farms, enabling thousands of people to use modern appliances. Clean water became widely available, and so did electricity. In India, the dam at Bhakra helped increase crop yields and double the income of agricultural laborers in the region. In 2000, a typical year, Bhakra produced thirty million tons of the grain purchased by Indian government agencies—eighty-five per cent of the total.

Dams have made it possible for the United States and Australia to store five thousand cubic metres of water per person. Middle-income countries like Morocco, Mexico, and China each store about a thousand. The per-capita figure for India is two hundred cubic metres—not much better than that for the poorest countries in Africa. Without sufficient water storage, irrigation becomes nearly impossible, and the relationship between irrigation and prosperity is absolute: if your land is fed by water, you are far less likely to be poor and far more likely to be educated.

In the past few decades, however, large dams have fallen out of favor in many places. One reason is that sixty per cent of the world’s biggest rivers have already been dammed. But public opposition to dams has been growing for years, and in 2000 the World Bank joined the World Conservation Union to publish a definitive study of their value and impact. It was a remarkable decision on the part of both groups, since the bank has played a central role in developing dams, and the Conservation Union, based near Geneva, has often expressed doubt that they are worth the money or the ecological and human disruption they cause. The groups’ joint report was thorough and largely negative. While “dams have made an important and significant contribution to human development, and benefits derived from them have been considerable,’’ it stated, “in too many cases an unacceptable and often unnecessary price has been paid to secure those benefits, especially in social and environmental terms, by people displaced, by communities downstream, by taxpayers and by the natural environment.” The report found that often, despite investments of tens of billions of dollars, dams do not achieve their goals for irrigation, power generation, or flood control. In the twentieth century alone, dams displaced as many as eighty million people, in addition to destroying forests and decimating fisheries.

Today, India has at least three thousand large dams and a thousand more under construction. The most bitterly opposed of them lie along the Narmada River in the state of Gujarat, which borders Pakistan and the Arabian Sea. These dams were conceived in the nineteen-forties, but construction didn’t begin for thirty years. When the Narmada project is finished, the dams are supposed to bring irrigation to more than eighteen thousand square kilometres of drought-prone land. But many local residents will be flooded from their homes, and activists, infuriated that the government has offered little in the way of compensation, have chained themselves to boulders and gone on hunger strikes in an effort to stop construction. Narmada has set off a national debate, not just over dams but over the environmental future of the country, as well as the conventional view of progress.

By far the most eloquent and extreme voice of opposition has been that of Arundhati Roy, the author of the Booker Prize-winning novel “The God of Small Things.” “For over half a century, we’ve believed that Big Dams would deliver the people of India from hunger and poverty,’’ she says. “The opposite has happened.’’ In 1999, Roy published an inflammatory and highly influential essay, “The Greater Common Good,” in which she argued that the most important of the Narmada dams, Sardar Sarovar, had raised doubts about the nature of Indian democracy. “Big Dams are obsolete,’’ she wrote. “They’re a Government’s way of accumulating authority . . . a brazen means of taking water, land and irrigation away from the poor and gifting it to the rich.’’ The issue is so controversial that in April, when Aamir Khan, one of India’s best-known movie stars, appeared at Sardar Sarovar to say that the government should do more to help the people it is displacing there, theatre owners in Gujarat responded by refusing to show Khan’s most recent film, “Fanaa.” Khan, who says that he is opposed not to the dam but only to the way local residents have been treated, has been denounced by state officials, and effigies of the actor have been burned. Although Prime Minister Manmohan Singh released a statement defending Khan’s right to speak out, leaders in Gujarat have demanded that he apologize for his public stance. Khan has refused. “I want the people of Gujarat to get water,” he said. “I love the people of Gujarat . . . but there should be justice for the displaced people, too.”