At first gulp, it seems there's plenty of water on the planet. In fact, there are about 335 million cubic miles of it, or enough to cover the United States to a depth of 93 miles, by one estimate.

But only 2.5% of it is fresh water, and most of that is locked up in glaciers or stored in rocks too far underground to reach.

Welcome to the harsh reality of water as a limited resource, where the motto is: "Use it and lose it." And we are.

Since 1940, global use of fresh water has roughly quadrupled, driven by population growth and heavier demand by each person, according to Population Action International. Already, lack of water hurts economies and human health in 20 nations, mostly in Africa and the Mideast; by 2025, at least 10 more may join the list.

"Sustaining Water: Population and the Future of Renewable Water Supplies," by Robert Engelman and Pamela LeRoy, outlines the problem and ventures some forecasts.

Renewable Water

Human survival depends on renewable fresh water--the tiny portion that is renewed, within a reasonable time span, by the Earth's natural hydrologic cycle.

Each year, the cycle deposits about 113,000 cubic kilometers of rain and snow on continents and islands. But most of it evaporates back into the atmosphere. Only 41,000 cubic kilometers, or 10,000 cubic miles, is leftto replenish aquifers or return to the oceans.

If all the world's water fit into a bathtub, the portion that could be used sustainably in any given year would barely fill a teaspoon.

The Hydrologic Cycle 1) Sun's rays draw water up from oceans through evaporation. 2) Water falls back to Earth as rain or snow. 3) Water seeps into ground and collects in rivers and lakes. 4) Water flows back to oceans and begins cycle again.

Who Uses It

The world averages at right masks vast differences. In dry areas where crops get heavy irrigation, agricultural use rises--to 80% in California, about 99% in Afghanistan and Sudan. Industry demands up to 85% in Belgium but less than 5% in many developing nations. In Kuwait and Zambia, which have little agriculture or industry, nearly two-thirds of the water goes to households. Agriculture: 69% Industry and energy: 23% Domestic/household use: 8%

The Driest Nations

Nations are water-scarce if the total availability drops below 1,000 cubic meters per person per year, according to a system devised by Swedish hydrologist Malin Falkenmark. At that level, economic development and human health are hampered.