Water. As human beings, we have an intimate and complex relationship with this liquid molecule composed of two parts hydrogen and one part oxygen. We can't live without it. We actually are it; our bodies are about 60 percent water. Yet we can drown by inhaling less than a cupful of the stuff. Tumbling over Victoria Falls, it represents one of the seven wonders of the world. But it cascades down our sink drains without provoking a reverent thought.

The liquid wealth of the planet, in the form of drinkable water (also known as potable water), is starkly unevenly distributed. Water covers nearly 70 percent of the Earth's surface, but just a scant 2.5 percent of that is freshwater, most of it locked up in glaciers and in the polar ice caps. The primary sources of freshwater are underground wellsprings (also called aquifers), surface waters (such as lakes, rivers, and streams), and rainfall. About 17 percent of our planet's population of 6.5 billion doesn't have access to clean drinking water, and most of these people live in developing nations. Some 5.1 million people die every year from waterborne diseases, many of which stem from lack of sanitation and the resulting water pollution.

Over the next hundred years, as we add a predicted three billion more people to the planet (and as climate change brings droughts, floods, and rising sea levels), water will give rise to some of society's toughest challenges-and, it's likely, to our bitterest wars. How we respond-measured in terms of formal water policies, treaties, and conservation efforts, as well as in terms of compassion and equity-will reflect an epic struggle between the best and worst impulses of humanity.

Who owns water?
In ancient times, we invented the gods Osiris and Triton (Egyptian and Greek, respectively) to rule water. Today, we leave that chore to the less mythologically satisfying courts, governments, and municipal water boards, who divvy it up. For the most part, the laws governing the right to ground or surface water for irrigation or household use invoke the principles of either adjacent land rights or what's called prior appropriation rights, which amounts to "I was here first." Most treaties and agreements have evolved from these basic principles, but with much more complexity around the question of who can divert what volume where. Not surprisingly, situations in which multiple communities, states, or countries share the same water source tend to be fraught and marked by dispute.

For example, California and Mexico have spent more than 10 years sparring in the courts over the All-American Canal, one of the world's largest irrigation channels. The canal, completed in 1942, carries water from the Colorado River to California's Imperial Valley, which lies between the Mexican border and the Salton Sea. About 7 percent of the total volume of water the canal transports seeps into the surrounding land. This caused flooding in the Mexicali Valley in Mexico until the community built infrastructure to harness the runoff. Now, years after a thriving community has grown up around that plentiful (though accidental) water source, California wants to put a liner in the canal. That move will effectively end the seepage, and thus cut off a key water source to the Mexicali Valley. California argues that the Mexicali Valley has merely benefited from a bit of engineering sloppiness, not from water Mexico is entitled to under treaty agreements. Mexico argues that negligently cultivating a community's dependence on a source of water for over 60 years engenders some responsibility to continue providing that water.

But the water "hot spots" most experts worry about are primarily in the Middle East, where several neighboring countries with hostile relations share access to waterways. Examples include the Jordan River, which is a primary water source for Israel, Jordan, Lebanon, and Syria. Meanwhile, Turkey is building a system of dams on the Euphrates in southeastern Anatolia. When complete, the system could cause Syrians to lose up to 40 percent and Iraqis to lose up to 90 percent of their water from the river. The fear is that these already tense relationships will flare into violent conflict if global warming significantly reduces the size of the prize.

"So far, even countries who have talked tough have returned to the bargaining table eventually," says Professor William Jury, an expert in soil physics and water management at the University of California, Riverside. "But the scary thing is that, in most cases, there are not formal agreements in place, particularly around how to handle division of resources if climate change were to lower the overall amount of water available."

How is water used?
By far the greatest use of water worldwide is for crop irrigation and other agricultural needs. Farming accounts for 66 percent of the water drawn from surface and ground sources, and 85 percent of the water "consumed"-meaning it no longer feeds back into the ground to replenish a water source. Industry accounts for 20 percent of water drawn, and municipal use, which includes your home's share, claims about 9 percent. The typical household pours 60 percent of its water on the lawn, rosebushes, and other keeping-up-with-the-Joneses vegetation, with the balance used indoors for showers, dishes, laundry, and flushing toilets.

In many regions around the globe, we are in the unfortunate position of consuming more water than is returned to the system through rainfall and "recharging," which is accomplished by pumping treated water back into groundwater reservoirs. Of course, this pattern is most acute in areas where there is little rainfall or persistent drought. Those of us in the Western world consume far more water (97,000 gallons per person annually), but tend to have more reserves to draw from, than subsistence farmers in Africa (who average 6,600 gallons per person annually, or less than 20 gallons per day).

If population growth is taken into account, our projected global need for water by 2050 (mainly for the crops to feed those people) is about 7,700 cubic kilometers more than we use annually now-an increase of 183 percent. Keep in mind that one cubic kilometer is more than 264 billion gallons of water. All that adds up to a need to think differently about water.

What's the biggest threat to our water supply?
There are three basic threats to the water supply globally, all of which can be mitigated by changes in behavior related to water planning and individual conservation.

Pollution: Contamination of surface waters and groundwaters is a growing problem everywhere. In developing countries, industrial waste is often less regulated, or legal repercussions are rare, so toxic sludge is returned directly to the environment. Another significant issue is that two billion people in poor nations lack sanitation services, which leads to waterways near urban squatter settlements or shantytowns turning into open sewers. In the West, roadway runoff containing petroleum and other chemicals seeps back into groundwater. Globally, the pesticides used in agriculture are finding their way into water supplies as well. In 1994, the U.S. Congress estimated that more than $75 billion would be required to clean up the 4,500 non-federal locations on the Superfund list, the Environmental Protection Agency's index of toxic sites. And experts have estimated that 20 percent of China's river waters are too polluted to be used to irrigate farms, let alone purify for drinking water.

Overdraft: This term refers to the practice of taking more water out of the natural water ecosystem than is replaced by rainfall or returned after treatment. More than 500 million people live in areas of persistent overdraft, meaning that they're writing water checks on an account that will soon be bone-dry empty. Better water management around agricultural use will help, as will individual conservation in these locales. However, one difficulty of enforcing any coordinated policy is that overdraft typically occurs in rural areas where individual farmers are struggling to scrape a living from the land, rather than on large, adjacent commercial farms, where shared irrigation systems can be more easily constructed.

Climate change: The soundest science predicts that global warming will likely bring with it increased flooding in coastal areas, rises in sea levels as polar ice melts, and additional drought in arid regions. That last problem is of most concern as it relates to water supplies for drinking and farming.

"We really haven't seen any phenomena like this in our history," Professor Jury says. "Potentially, we have up to three billion people living in water-scarce areas that are also poor nations. If you look at the projections for mid-century, we are headed for a huge food challenge."

What are the potential solutions?
According to Jury, our best solutions are already within reach: better water management practices, more conservation, and better pollution policing. He points to models such as California, where droughts in the 1970s led to widespread conservation and better technologies for increasing the efficiency of household use. Today, for example, the metropolitan Los Angeles area uses 20 percent less water than it did then, despite a substantial population increase.

While technologies that pull salt out of ocean water are of great interest in the scientific community, the means to make that process affordable on a large scale does not yet exist. Rather than a technological silver bullet, Jury suggests that small-scale innovation will shape the "next revolution." In particular, he says, strategies that help subsistence farmers use water more efficiently will be the way to save lives.

"But the greatest cure for the water crisis is to conserve more," Jury says. "It's just not good policy to waste water, no matter where you are."

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