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Scarcity of freshwater is an increasingly critical public health problem in many parts of the world. World leaders, including United Nations Secretary-General Ban Ki-moon, have urged that this issue be given high priority. Inadequate access to safe freshwater contributes to waterborne disease, malnutrition, poverty, economic and political instability, and conflict- potentially violent conflict- between countries or groups within countries.
Approximately 97.5% of all water is either salt water or water that has become polluted. Of the remaining 2.5%, nearly 70% is frozen in glaciers and the polar ice caps. Less than 0.01% of all water worldwide is available for human use in lakes, rivers, reservoirs, and easily accessible aquifers.
About three fifths of water flowing in all rivers is shared by two or more countries-in 263 river basins in 145 countries, where two fifths of the world's population lives.1 As a result, many countries are highly dependent on water resources that originate from outside their national territory. For example, 34% of water resources in India and 76% of water resources in Pakistan originate from outside these countries.2 As another example, the Nile River Basin is shared by 11 countries that are mutually dependent for their water resources.
The World Bank estimates that people generally require 100 to 200 liters of water daily to meet basic needs (36.5-73.0 m3 of water per person annually). If one includes other uses of water, such as agriculture, industry, and energy production, the total annual average requirement of water per person is 1000 cubic meters.3 In 1990, 11 countries in arid or semiarid regions of Africa and the Middle East had less than 1000 cubic meters of freshwater available per person.3 Given anticipated major population increases, each of these 11 countries will have substantially less water per person in 2025.
One billion people do not have access to safe water-a problem that will likely increase as the world population grows from 6.8 billion people now to about 9.0 billion by 2050. This problem likely will become especially severe in countries with high population growth rates that share a major source of freshwater with other countries.4
Conflicts over water, both within countries and between countries, are sharply increasing (Table 1). However, few of these conflicts have led to violence. Major underlying...