The world is running out of usable water, and there is no substitute for it, says Sandra Postel, director of the Global Water Project in Amherst, Massachusetts, and co-author of the book "Rivers for Life." There are energy alternatives for oil, she says, but water is unique and yet strangely disregarded by much of the world as if it will always be with us.

"With water there is a tendency to think that it is a limitless resource because it falls from the sky. We can count on water coming back year after year. What is harder for people to understand is that, yes, it is renewable. It does fall from the sky year after year but only in a finite amount. The problem we are in now is that our use of water is bumping up against the limit of that finite quantity in more and more parts of the world."

No part of the world is really safe from a water shortage, says Sandra Postel, but some especially hot and dry areas are in deep trouble right now with the signs all too visible:

"We have many major rivers that are now no longer reaching the sea for long periods of time throughout the year. We have ground water being over-pumped in many parts of the world Middle East, North Africa, the north plain of China, India, parts of the western United States. These are very clear signs that we are over tapping the water supply even as demands continue to rise in many of these areas."

What is to be done? All kinds of things. There is no one answer, says Peter Gleick, director of the Pacific Institute in Oakland, California. Remedies vary according to time and place:

"We are going to have to look to new technology like desalination and the use of reclaimed and recycled water. We are going to have look to new technology to improve the efficiency of water use. We are going to have to look at the economics to make sure we pay the right price for water and that we do not waste it."

Desalination making ocean water drinkable, is under way, especially in the arid Middle East. Think of the ocean as the world's largest reservoir, say proponents. But desalination is energy intensive and costly. Thus, it is not the solution for developing countries, notes Sandra Postel. Better water management is the answer:

"We need to be much more efficient and conserve water in a much more systematic and dramatic way than we have in the past. We have continuing population growth, continuing consumption growth against a finite water supply, and that is a no-win situation. For example, in the western United States where we have a terrible drought right now, in many places half the water that is being used in the home environment is outside to water landscapes that really do not belong in a desert environment."

This heedless waste of water is a large part of the problem, says Mr. Gleick. Some Americans insist on bringing eastern habits to the desert west. They are at odds with their environment and imperiling their water supply:

"We do not use water wisely there. We are growing very water-intensive crops in some parts of the southwest, using the vast majority of the water in the Colorado River, for example. Another problem is we still have this fixation in the United States for green lawns. Now green lawns are perfectly reasonable in the eastern part of the United States. But in the west, they use a tremendous amount of water, and they are really not suited for the climate."

Just look at all the grass-covered golf courses lining the western desert, says Mr. Gleick, and think of the water used to maintain them.

Golf courses, to be sure, are not the problem in developing countries, but they have their own waste inadequate structures that leak water and poor management that contaminates it. Since agriculture accounts for 70% of global water use, the kind of crops that are planted are crucial. Growing rice, for example, requires much more water than, say, wheat.

As people flock to cities, they tend to dump more toxic chemicals along with human waste into their waters, leaving still less than is usable. And not just in poor countries, says John Janssen, senior scientist at the Great Lakes Water Institute at the University of Wisconsin in Milwaukee:

"In Milwaukee in 1993 some bad bacteria washed out into Lake Michigan and got sucked into the water treatment plant here, and people died from it. I got terribly sick for a couple of weeks from it myself. I guess the fact that it happened 100 years ago in Chicago and only about ten years ago in Milwaukee tells you that it can happen just about anywhere, probably everywhere. It is just that some places are more vulnerable than others."

Mr. Janssen says some countries are keenly aware of the looming water crisis. Middle Easterners even considered towing icebergs from northern waters to their shores, then thought better of it.

In general, says Mr. Janssen, governments are slow to act until their people insist on it. Such is the case in the United States:

"They take seriously what the public takes seriously and turns into a campaign issue. If the public is not taking it seriously, the politicians for the most part do not which can be a severe problem if you have a national water policy that is made in Washington and you have just a few states that have a problem. It can be pretty much ignored."

What will it take to wake up people? Probably, some crisis, says Mr. Janssen, and let's hope it is a minor one that limits drinking water but does not cost lives. A major one might be unthinkable. So now, he says, is the time to do some serious thinking about how to preserve the earth's endangered water supply.