Scientists who once looked to the clouds for more water are increasingly looking downward for new sources. What's tempting them is deep underground aquifers -- huge rivers and lakes far beneath the surface, some of them containing "fossil" water as much as a million years old.
Recent mapping efforts suggest that some of these aquifers hold enough water to support billions of people for centuries. But the lean and thirsty looks engendered by that enormous wealth of water have made some hydrologists, economists and political scientists nervous.
Little is known about the ecological impact of deep aquifer pumping, especially since it's still not clear which of these sources are naturally refilled over time and which are true fossil aquifers -- meaning they exist in sealed spaces much like oil reserves, available for one-time consumption and then lost.
Moreover, of the hundreds of water treaties and shared-use agreements forged by nations in recent decades, none applies to underground aquifers. With major aquifers crossing international boundaries, the potential is rising for conflict.
Only 2.5 percent of the world's water is fresh, and the vast majority of that is frozen in glaciers and icecaps. All told, less than three-tenths of 1 percent of the planet's fresh water is in the lakes and rivers that have served as the major sources of water through most of human history, and much of that is drying up or becoming spoiled.
"There are all kinds of signs that this level of use is not sustainable," said Sandra Postel, director of the Global Water Policy Project in Amherst, Mass., pointing to falling water tables in many parts of the world. Already, about 8 percent of the food that feeds the world's 6 billion people is being grown by taking water that is not being replenished. "If that's the case now," Postel said, "what are we going to do when we need to feed 8 or 9 billion people?"
One answer is to go underground, where there is 100 times the amount of water found on the surface -- much of it at depths of a half a mile or more.
Until recently, it wasn't worth trying to get at that water -- a cubic yard of it weighs about a ton. But with shortages now looming globally that equation is changing.
"The water in these aquifers is of much better quality nowadays than in almost any of the rivers or lakes," said Alice Aureli, head of the United Nations Educational, Scientific and Cultural Organization's transboundary aquifer project. "It's an invisible resource. But it's most probably the resource our children will depend on."
Armed with seismic and core-drilling technologies long used by the oil industry, and with funding from several international agencies, the International Association of Hydrogeologists has begun a massive underground mapping project. It hopes to determine the outlines and volumes of the world's larger aquifers -- the first step to working out agreements among the countries that share them.
Among the bigger bodies of water under study are the Guarani aquifer, shared by Argentina, Brazil, Paraguay and Uruguay, which could provide 27 gallons of water a day to 5.5 billion people for 200 years. Another is the Kalahari/Karoo aquifer, shared by Namibia (the driest nation south of the Sahara), Botswana and South Africa. Others underlie the Middle East, North Africa and the Caucasus.
Time is of the essence if political skirmishes are to be avoided, as evidenced by recent events at the Nubian Sandstone Aquifer, which lies beneath the sands of Libya, Egypt, Chad and Sudan. The aquifer is a remnant of rains from 10,000 years ago, when the Sahara was green and lush. Its volume is about 500,000 times the annual flow of the Nile, but only scant amounts are added annually in the current climate.
Now, having completed one of the largest civil engineering projects on earth, the Libyan government is drawing hundreds of millions of gallons of water per week out of the reservoir and pumping it more than 2,000 miles north through subway-tunnel-sized pipes to Tripoli. That has the other countries overlying the aquifer wondering whether they'll have a chance to get their share.
Israelis and Palestinians have chafed for years over how to share the Mountain Aquifer, which lies beneath the West Bank. Replenishment by rainfall is mostly from the Palestinian side, but Israel draws about 85 percent of the aquifer's annual yield.
Shammy Puri, chairman of the international commission coordinating the global aquifer survey, hopes the project can defuse such tensions. "This is a resource that is the heritage of mankind," said Puri, who is pulling together lawyers, economists, political scientists and legislators from affected countries to start talking to one another. But aquifers work on longer time scales than people are used to, he said.
"When you take water from a river, the impact is immediate. But by the time groundwater extraction is felt, it can be tens of years or even hundreds of years," and too late to fix the hydrologic and political damage, Puri said. "If you're going to extract, and it's going to affect your neighbor, you need to take that into account."
"A lot of people like to talk about water as a source of conflict, but we want to see it differently, as a solution to conflict," said Manuel Dengo, chief of water resources for the United Nations. "If you are sharing water with your neighbor and using the same pipeline, you are really connected."
Aaron Wolf, a geoscientist at Oregon State University in Corvallis, also sides with the optimists. "The politics of international water is an encouraging story," he said, noting that about 400 treaties for surface water have been signed in the past 50 years. "Even in the Middle East, the last time there was direct violence over water was in 1970."
In fact, the only all-out water war in recorded history was fought 4,500 years ago, Wolf said, between the Umma and the Lagash of Sumeria. The clay-tablet peace treaty they later agreed to is on display today in the Louvre.