The United States, because it's rich enough to engineer its way around most water problems and buy some breathing room for those it can't solve, is actually in relatively good shape. Other countries have more immediate, less easily delayed, problems. For example, India's "green revolution" is by most standards one of the past century's great success stories. Once a country where tens of millions of people existed on the edge of starvation, it is now nearly self-sufficient in food, thanks largely to the irrigation of vast fields of rice, alfalfa, sugarcane, and corn. Because India's rivers weren't up to this task, its farmers and villagers have gotten the necessary water by digging wells, millions of them, to tap what once seemed like an inexhaustible underground aquifer. But now, predictably, underground water levels are dropping, rendering old wells inadequate and forcing farmers to dig deeper and use more electricity to pull water to the surface. As municipal wells run dry, towns are bidding for water on the open market, and for many farmers, it's now more profitable to sell water to distributors than to use it to grow crops. Meanwhile, in some parts of India, as water levels drop, fluoride-which occurs naturally in the granite rocks that underlie much of the country-has begun to contaminate wells, and literally millions of people are now suffering from various kinds of fluoride-related bone growth deformities. And-this being India rather than Las Vegas-there's no immediately apparent fix. The groundwater is poisonous, the surface water is polluted by human and industrial waste, and filtering or otherwise treating either water source is prohibitively expensive. In many parts of India-and China and the rest of Asia-the green revolution is about to be replaced something else, as yet unnamed but far less pleasant.

A few more examples:

• In the southern African state of Swaziland, the rainy season used to start in September but lately has been coming in October or November and dropping considerably less rain. Water levels in the Maguga Dam, the country's largest reservoir, were at one-third of capacity in 2007, causing the government to impose water rationing on its citizens.

• In Australia, the worst drought in recorded history recently ended, but water levels in rivers and reservoirs remain low. Restrictions on water use, hitting primarily farmers, are projected to remain in place for years.

• The Andes glaciers that supply most of Peru's water and electricity are disappearing, leaving the growing population along the country's desert coast with increasingly precarious water and power supplies.

The list could go on, but the point is clear: Rising populations are bumping up against a shrinking supply of fresh, clean water. The International Water Management Institute estimates that about a fifth of the world's population, or more than 1.2 billion people, already live in areas with insufficient supply. The UN expects that figure to rise to fully two-thirds of the world's population within 20 years. The result will be ugly, which is to say full of opportunities. One of the most interesting is desalination. Read about Desalination.