"You look at Earth from outer space, you see this big blue sphere. You don't really think of it as a scarce resource, or think about how water -- fresh water -- can impose restrictions on human activity," Stanford University biologist Gretchen Daily said in a telephone interview this week.
Only about 2.5 percent of the Earth's water is fresh, and two-thirds of that is locked up in glaciers and ice caps.
So less than one percent of all the Earth's water is fresh water in aquifers, rivers, soil, lakes, swamps, plant life and the atmosphere, said the team made up of Daily, Stanford colleague, scientist and author Paul Ehrlich and Sandra Postel of the Global Water Policy Project in Cambridge, Mass.
And not all that water is easily accessible. Where there is lots of water, like in the Amazon River basin or the far north tundra, there are few people. And in areas hospitable to human habitation, water is not necessarily abundant.
The possibilities of capturing more of the Earth's available fresh water are limited.
About 26 percent of the water available through rain is already captured, and there is not much more land available for rain-fed farming and grazing, they wrote.
Just over half the water from runoff, in lakes and streams, is already captured. Building dams could increase that by about 10 percent in 30 years, but the global population is forecast to grow by about 45 percent during that period, they wrote.
Pressures on water supplies are already being seen in declining fresh water fish populations. But more problems can be expected in food supplies, which in turn can create more international conflicts, immigration pressures and public health problems, Daily said.
In "ecological superpowers" like the United States and in struggling poor nations, "We've got to bring human activity into balance with nature's bottom line," she said.
For instance, instead of using up water to clean up pollution, she advocated less pollution. Better irrigation and farming technology and ending subsidies that keep water artificially cheap were other remedies, she said.
"Some regions are already facing constraints," she said. "The timetable for action is starting now. You can't change these things overnight."