Water, used by households, agriculture, and industry, is clearly the most important good provided by freshwater systems. Humans now withdraw about one fifth of the world’s rivers’ base flow (the dry-weather flow or the amount of available water in rivers most of the time), but in river basins in arid or populous regions the proportion can be much higher. This has implications for the species living in or dependent on these systems, as well as for human water supplies. Between 1900 and 1995, withdrawals increased by a factor of more than six, which is greater than twice the rate of population growth (WMO 1997).

Water supplies are distributed unevenly around the world, with some areas containing abundant water and others a much more limited supply. In water basins with high water demand relative to the available runoff, water scarcity is a growing problem. Many experts, governments, and international organizations are predicting that water availability will be one of the major challenges facing human society in the 21st century and that the lack of water will be one of the key factors limiting development (WMO 1997).

These maps show water supply per person for individual river basins as of 1995 and projections for 2025. Water experts define areas where per capita water supply drops below 1,700 m3/year as experiencing “water stress”—a situation in which disruptive water shortages can frequently occur. In areas where annual water supplies drop below 1,000 m3 per person per year, the consequences can be more severe and lead to problems with food production and economic development unless the region is wealthy enough to apply new technologies for water use, conservation, or reuse. This map is based on the analysis carried out by WRI for the Pilot Analysis of Global Ecosystems: Freshwater Systems (PAGE). The first map shows that as of 1995, some 41 percent of the world’s population, or 2.3 billion people, live in river basins under water stress, with per capita water supply below 1,700 m3/year. Of these, some 1.7 billion people reside in highly stressed river basins where water supply falls below 1,000 m3/year

In the second map we see water scarcity projections for 2025. The analysis shows that by 2025, assuming current consumption patterns continue, at least 3.5 billion people— or 48 percent of the world’s projected population —will live in water-stressed river basins. Of these, 2.4 billion will live under high water stress conditions. This per capita water supply calculation, however, does not take into account the coping capabilities of different countries to deal with water shortages. For example, high-income countries that are water scarce may be able to cope to some degree with water shortages by investing in desalination or reclaimed wastewater. The study also discounts the use of fossil water sources because such use is unsustainable in the long term.

In the second map, a selected number of basins have been outlined. These watersheds represent basins that are in or approaching water scarcity and where the projected population for 2025 is expected to be higher than 10 million. Six of these basins including, the Volta, Nile, Tigris and Euphrates, Narmada, and the Colorado River basin in the United States, will go from having more than 1,700 m3 to less than 1,700 m3 of water per capita per year. Another 29 basins will descend further into scarcity by 2025, including the Jubba, Godavari, Indus, Tapti, Syr Darya, Orange, Limpopo, Huang He, Seine, Balsas, and the Rio Grande.