Water: A finite resource






Although water covers 75 percent of the world's surface, 97.5 percent of the earth's water is salt water; of the remaining 2.5 percent, most is locked away as groundwater or in glaciers.

All life depends on water. Although overall the world's population can count on a supply of about 9 000 cubic kilometres of freshwater per year, it is not evenly distributed. Hydrologists regard countries where indigenous water supplies average less than 1 000 cubic metres per person per year as "water-scarce".

Finding enough water can be a formidable task. Water-scarce countries must balance strategies for increasing water supply against risks of upsetting the delicate balance of the local ecosystem.

Without water our planet would be a barren wasteland. Of the three main ways in which people use water - municipal (drinking water and sewage treatment), industrial and agricultural (mostly irrigation) - farming accounts for the largest part, some 65 percent globally in 1 990.

Water is a finite resource: there are some 1 400 million cubic kilometres on earth and circulating through the hydrological cycle. Nearly all of this is salt water and most of the rest is frozen or under ground. Only one-hundredth of 1 percent of the world's water is readily available for human use.

This would be enough to meet humanity's needs - if it were evenly distributed. But it is not. In Malaysia 100 people share each million cubic metres of water; in India, the figure is 350 and in Israel, 4 000. And where there is water, it is often polluted: nearly a third of the population of developing countries has no access to safe drinking water.

In many countries, the amount of water available to each person is falling, as populations rise. By the year 2000, Latin America's per caput water resources will have fallen by nearly three-quarters since 1950. In the twenty-first century the main constraint on development in Egypt will be access to water, not land. Over 230 million people live in countries - most of them in Africa or the Near East - where less than 1 000 cubic metres of water is available per person each year.

Even countries with greater supplies are extracting too much from their underground water reserves. The water table under Beijing is sinking by 2 metres every year, while Bangkok's has fallen by 25 metres since the 1950s. The level of the vast Ogallala aquifer, which lies beneath eight US states, is dropping by nearly 1 metre a year.

Pollution exacerbates the problem. Some 450 cubic kilometres of waste water are poured into the world's surface waters every year: two-thirds of the world's available runoff is used to dilute and bear it away.

A world short of water is also an unstable world. More than 200 river systems cross international boundaries, and 13 rivers and lakes are shared by 96 countries. Over-use or pollution by countries upstream can be devastating for those downstream. Access to water, particularly in areas where rainfall is low or erratic, is becoming a major political issue and vital to national interests.

Faced with these crises, the world must learn to be less wasteful and to manage its water resources better. Methods include conserving supplies, using reservoirs and small dams to catch runoff, recharging aquifers, protecting watersheds and recycling waste water in agriculture and industry.


Where the water is

Distribution of the world’s water
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The hydrological cycle

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The world’s hydrological cycle links lakes, soil moisture, rivers and biological systems. This great water pump causes some 113 000 cubic kilometres to fall as rain and snow every year


Annual water use

Estimated annual world water use


Water availability and water scarcity

Water availability by region

Sources: FAO; The World's Water Assessing The Resource

Water-scarce countries (based on less than 1 000 cubic metres available per caput per year)

Since 1955
Bahrain, Barbados, Djibouti, Jordan, Kuwait, Malta, Singapore

Since 1990
Algeria, Burundi, Cape Verde, Israel, Kenya, Malawi, Qatar, Rwanda, Saudi Arabia, Somalia, Tunisia, United Arab Emirates, Yemen

By 2025*
Comoros, Egypt, Ethiopia, Haiti, Iran, Libya, Morocco, Oman, South Africa, Syria
**Peru, Tanzania, Zimbabwe

* on UN population projections
** on medium projections
*** on high projection only



Irrigation losses

Irrigation systems have existed for almost as long as settled agriculture. Five thousand years ago, the ancient Egyptians used the waters of the Nile to irrigate their crops. Two thousand years later, the great civilizations of the Fertile Crescent, stretching from the eastern Mediterranean to the Persian Gulf, were built on irrigated agriculture.

Irrigation is essential to feeding the world. Although only 17 percent of the world's cropland is irrigated, it produces over 33 percent of our food, making it roughly two and a half times as productive as rain-fed agriculture.

In spite of the pressing need for expansion, less new land is now being brought under irrigation than in the early 1970s. This is because of the shortage of suitable land, the rising cost of constructing irrigation systems and the scarcity of water itself.

Bureaucratic interference, faulty management, lack of involvement of users, interrupted water supplies and poor construction have all led to poor performance, which has discouraged investment. In some cases up to 60 percent of the water withdrawn for use in irrigation never reaches the crops.

In addition, waterlogging and salinization have sapped the productivity of nearly 50 percent of the world's irrigated lands. Unless irrigated fields are properly drained, salts can build up in the soil, making the land infertile. Salinity affects 23 percent of China's irrigated land and 21 percent of Pakistan's.

Other problems include the accumulation of pollutants and sediments in large dams and reservoirs, and the fact that irrigation systems provide an ideal habitat for the vectors of waterborne diseases.

The key to improved irrigation lies in more efficient use of water; recycling waste water and proper drainage. Drip irrigation and low-pressure spray systems are now being used in over 20 countries to deliver water directly to crops. Small dams, located closer to agricultural areas, are replacing large ones. Canals are being lined with concrete and covered to reduce seepage and evaporation. Several countries now use treated waste water for irrigation; Israel was using up to 30 percent of its urban waste water in this way as early as 1987.

Overhead pivot spray

Irrigation canal

Irrigational flooding


Percentage of land irrigated

Irrigated land, 1993
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Land under irrigation


Salt of the earth threatens production on irrigated land

FAO estimates that salt buildup has severely damaged about 30 million of the world's 237 million hectares of irrigated farmland. As much as 80 million hectares more are affected to some degree, with about 1.5 million hectares of irrigated land lost each year to waterlogging and salinity.

Sources: FAO, National Geographic

Heavily salted
Salinity affects as much as one-quarter of the irrigated land in some countries:
Country Percentage salinated
Mexico 10
India 11
Pakistan 21
China 23
United states 28