Postel says that looming water shortages in many parts of the world have the potential to spark domestic instability and international conflict. She has a forthcoming book to be released later this year entitled, "Pillar of Sand: Can the Irrigation Miracle Last?" Postel was interviewed by Charlene Porter.

Question: How do diminishing water supplies threaten social and political stability?

Postel: Some of the basic indicators of water problems and issues related to water supply -- both physical and numeric indicators based on population and water availability -- suggest that there are signs of trouble with regard to meeting all the demands for water we see coming ahead. If water shortage ends up, for example, causing food prices to increase, we know that rising food prices are a sign of social instability. We've seen that in a number of places just over the last few years -- when governments, for one reason and another, have had to remove subsidies for food, we've seen riots in the streets in a number of countries. So anytime there's an increase in food prices in a poor country, you risk social instability. We've seen it happen in countries like Indonesia and Jordan; there were problems in India this year just from the price of onions.

The other clear sign of political instability has to do with what's occurring from increasing water demands in water scarce river basins, where rivers are shared by two or more countries. A number of hot spots of potential water dispute exist where populations are continuing to increase rapidly, and where there's no treaty yet in place that determines how the river water should be shared by those countries.

The Jordan Basin, the Nile Basin, the Tigris Euphrates river basin, the Amu Dar'ya and Syr Dar'ya rivers in the Aral Sea basin in Central Asia -- these are areas where the demands for water are increasing. If you add up the estimated demands for river water in each of those areas, you find that they typically exceed the amount of water in the river.

For example, in the Nile basin, there is no way that Ethiopia, Sudan, and Egypt can all achieve their irrigation goals; there is just not enough water to go around. And there is as yet no water sharing agreement, no treaty, that includes all the key countries that sets out how that water should be shared. With populations expected to grow in each of these river basins by between 40 and 70 percent over the next 30 years, there will be increasing competition for a limited amount of water.

Q: Against that backdrop of potential instability, let's examine some of the solutions for effective water conservation and improved efficiency that you've studied. Let's start with the process of desalinating water. In one of your previously published articles, you wrote that some Arab nations are turning oil riches into water. What are the inherent long-range problems you see with desalination?

A: The cost. Desalination is very energy intensive. It takes a lot of energy to remove salt from water. It's an expensive source of supply, and that's why you tend to find it in places where energy is readily available and relatively inexpensive. That's why I made the comment about turning oil into water, because very few other places can afford to do that. About half of all the desalination capacity is in the Persian Gulf region. At the moment, we're getting less than two-tenths of 1 percent of global water use from desalination, so it's a very small share.

I would see that share increasing because the costs are coming down and water constraints are becoming more severe. So the combination of improving technologies and rising costs of water would suggest to me we will see more desalination, but I still think, for the foreseeable future, it will be a relatively minor source of supply just because it will remain too expensive.

There's no way you could imagine irrigating with desalinated water. It's just way out of line with what farmers could afford to pay. And, of course, irrigated agriculture is far and away the biggest user of water. So I still see it as a last resort supply for drinking water, and not something that's going to really save us in terms of dealing with the seriousness of the water problem.

Q: Let's turn to water conservation issues in urban areas. Obviously, urban areas around the world have diverse situations as far as water supply goes, but generally speaking, what do you see as being some of the most effective techniques on the horizon for trying to encourage water conservation and more efficient use of supplies in major urban areas?

A: This is a big challenge. Many of these cities in developing countries have 10 to 20 million people. Mexico City has 15 million people. And it's very difficult to provide the infrastructure to supply that many people in a concentrated area with water; then equally difficult to collect the wastewater from all those households; then treat it; and then release it to the environment.

The infrastructure challenge with urban water supply is huge and very much of it is still unmet. Especially in the rapidly growing mega cities. You've got so many people in a concentrated area, and finding enough water within a reasonable distance of the city to supply that many people is very difficult. That's one part of the challenge. The other part is that, unlike irrigated agriculture where the water doesn't have to be of super-high quality, you need to have treated water, and then you need to somehow deal with the wastewater. This involves treatment plants and pipes and all kinds of expensive infrastructure.

So just keeping up with the rates of growth we're seeing in these cities is a big, big challenge. We've got about 2,500 million people living in cities now. And that's projected to double by 2025. There will be an estimated 5,000 million people in 2025. This is a really big challenge.

Many of the mega cities are having a difficult time with water supply and wastewater. If you look at most of these cities, they're not treating more than 10 percent -- at the most 20 percent -- of their wastewater. Most of the wastewater is still being released to the environment untreated in these mega cities. The rivers flowing through these areas are very, very polluted. Water quality is deteriorating as a result of industrial and municipal wastewater contamination. It's sort of a double whammy -- you've got a water supply problem to begin with, and then you end up polluting some of the supplies you do have, rendering them unusable. Water quality and water quantity problems go hand-in-hand in these areas.

To me, the real tragedy is that, in the urban sector, the very poorest people generally lose out completely because they don't have access to piped-in water. The very poorest members of these cities in developing countries often have to pay a significant share of their income to buy water from vendors because they don't have access to the public water supply. These are people in the shanty towns on the outskirts of cities. Some of them spend a quarter of their income paying for water from vendors who bring it in by truck twice a week or so. So there's a huge inequity here: society subsidizes the piped-in water for urbanites, but it's the very poorest people who don't have any piped-in water at all who end up spending a huge share of their income for water. A very big problem.

The one thing that's important in these cities is that they build an efficient infrastructure into the urban water systems from the beginning, that they use the most efficient appliances and fixtures at the household level that are available. Again, this is not generally being done.

In the United States, we now have a law that all new toilets, faucets, and showerheads that are manufactured have to meet a certain standard of efficiency. I think this type of policy would be very important in these rapidly growing developing countries, because it stretches the water supply right from the start. When you've got a household able to live with 30 percent less water than a neighboring one, that gives you more water supply to work with.

Q: Let's move on to water subsidy policies and how modification of those may encourage more conservation?

A: This is a big problem. Farmers in particular are getting heavy subsidies for irrigation water. It's difficult to find a situation where farmers are paying more than 15 to 20 percent of the real cost of the water. Generally, they're not getting enough incentive to use water efficiently in many cases. And so pricing water in a way that does give a better signal to farmers would be an important step in moving toward a more efficient use of irrigation water.

This is a complicated subject because in much of the Third World, water is not really delivered on demand as it is here. Water is sent through a canal and you take it when it's your turn. You don't have much of an option for using water differently. But where you do have water on demand, pricing structures can greatly influence how efficiently a farmer uses water.

I do think it would be difficult, overnight, to start charging the full cost of water. That would be very disruptive and could throw farmers out of business. Crop prices are very low. But there are various ways of structuring water prices to encourage farmers to use water more efficiently without causing an undue burden on them.

One of the programs that I've seen in California involves a pricing structure. This is in an irrigation district where they wanted to cut down on the amount of drainage that was causing contamination problems. What they did was price 80 percent of a farmer's prior use of water at the same level it had been in the past; but then they imposed a steep increase for the next 10 percent of overall usage; and a really steep increase for the last 10 percent. So it encourages farmers to try to reduce that use by 10 or 20 percent to avoid those steeper charges. It seemed to work. There are creative ways to do it without creating an undue burden on the farmer.

We've seen that where incentives exist, farmers do respond. Where they are able to, they do invest in more efficient technologies. We've certainly seen that, for example, in parts of Texas where the Ogalala Aquifer has been depleted -- farmers have put in more efficient irrigation systems and reduced their water use.

What I'm seeing now are that low-cost methods of drip irrigation, which is a very efficient way of delivering water directly to the roots of crops, have been developed that allow this technology to spread much more widely than what we would have earlier thought. There's certainly potential for the traditional drip irrigation systems to be used much more widely than they have been. In addition to that, these low-cost systems can now make drip irrigation available to small farmers, and poor farmers who often are the ones who have a scarce water supply to begin with and can benefit from technologies that allow them to spread that water more widely.

I visited some of these systems last year in the hill areas of the lower Himalayas in northern India, where farmers have water scarcity problems in the summer. They routinely said to me they would be able to double their crop land area if they had enough water. So drip irrigation allows them to take the water supply they do have and maybe get twice as much use out of it by allowing them to use it more effectively. These kinds of things have not been pushed that much; they're just starting to get attention.

Q: Besides drip irrigation, are there other new types of irrigation methods that may offer promise for the future?

A: Sprinkler technology is another good one that could be used more widely. Micro sprinklers in particular. Once again, I'm thinking of the small farmer. Particularly in developing countries, irrigation technology has generally been geared toward larger farms. The canal systems often deliver water to farmers on small plots, but where surface water is unavailable and farmers have to rely on ground water, they typically have had trouble accessing ground water for small plots because the technologies are too expensive. The micro irrigation technologies, the small-scale drip, and the micro sprinkler can benefit the small farmer, and they tend to be efficient ways of using water.

The micro sprinkler is not that different from a regular backyard sprinkler. You move it around six or seven or eight times on an acre of land to irrigate your crops. You can use it on a crop like wheat that isn't appropriate for drip systems, but it tends to be more efficient than a gravity-based system using flooding or a furrow-based system. You deliver less water more uniformly so you can water the crop with less water than with flooding furrows and ditches.

Improving efficiency in water supply systems overall is dependent on getting incentives right, and getting the institutions working better is a key thing. So is more farmer involvement, more accountability throughout the system, so that when farmers pay more for water they see an improvement in their system.

These kinds of things may sound sort of mundane, but they are so important to the way things work.