And harnessing this 'rather mysterious natural resource.'.

• Author: Seckler, David

A study by the International Water Management Institute indicates that by the year 2025 about one third of the population of developing countries - some 1.3 billion people - will live in areas of chronic water scarcity. This means that even if they fully develop their water resources, they will not have enough water to maintain even the 1990 per capita levels of use for agricultural, domestic, industrial and environmental purposes. In most cases, 1990 levels of per capita water use are already too low for adequate health, environmental quality and economic development. Many of the countries in this category will have to withdraw water from agriculture and rely on food imports to meet their needs. Some of them can afford this, many cannot. An additional 500 million people, mainly in sub-Saharan Africa, will experience acute water scarcity. They potentially have sufficient water resources to meet reasonable standards of per capita use, but the costs of actually developing this much water are likely to be prohibitive. Of course, as in all cases of extreme scarcity, poor people will suffer the most.

With this outlook, the task of conserving water through better technology, demand management and by other means becomes especially important. Since agriculture consumes over 80 per cent of all the developed water supplies of the world, people are naturally interested in conserving water in this sector. Most of the world's irrigation is by traditional gravity irrigation systems that have an average application efficiency (henceforth, unless otherwise noted, simply "efficiency") of only 45 per cent. In contrast, sprinkler, level basin and surge systems have efficiencies of around 70 per cent, and drip systems can attain efficiencies of 90 per cent. Much the same is true in the other sectors.

Low-flow toilets and showers are twice as efficient as conventional devices. The industrial sector has already made great advances in increasing efficiency, mainly to control water pollution. Because of technological and other changes, total water withdrawals in the United States has actually decreased over the past decade, while the economy has enjoyed unprecedented prosperity. These considerations have led many people to believe that with appropriate policies, such as pricing water at full marginal cost, sufficient water can be conserved and saved to meet most if not all future needs. But water is a rather mysterious natural resource where not all is as it might...