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World’s surface water: evaporation and runoff

Because much of the world’s surface water is far from concentrations of human settlements, not all of it is readily usable.
- It is estimated that the freshwater available for human consumption varies between 12,500 km3 and 14,000 km3 each year (Hinrichsen et al., 1998; Jackson et al., 2001).

- Many countries in Africa, the Middle East, western Asia, and some eastern European countries have lower than average quantities of freshwater resources available to their populations.

- Due to rapid population growth, the potential water availability for the earth’s population decreased from 12,900 m3 per capita per year in 1970 to 9,000 m3 in 1990, and to less than 7,000 m3 in 2000 (Clarke, 1991; Jackson et al, 2001; Shiklomanov, 1999).

- In densely populated parts of Asia, Africa and central and southern Europe, current per capita water availability is between 1,200 m3 and 5,000 m3 per year (Shiklomanov, 1999).

- The global availability of freshwater is projected to drop to 5,100 m3 per capita per year by 2025. This amount would be enough to meet individual human needs if it were distributed equally among the world’s population (Shiklomanov, 1999).

- It is estimated that 3 billion people will be in the water scarcity category of 1,700 m3 per capita per year by 2025 (UNEP, 2002).

- The uneven distribution of freshwater resources creates major problems of access and availability. For example:

Asia and the Middle East are estimated to have 60% of the world’s population (3,674,000,000 people in 2000), but only 36% of its river runoff - much of which is confined to the short monsoon season (Graphic Maps, 2001; Shiklomanov, 1999).

South America, by contrast, has an estimated 6% of the global population ( 342,000,000 people in 2000) and 26% of its runoff (Graphic Maps, 2001; Shiklomanov, 1999). These examples do not take into account groundwater abstraction.