More than half the global runoff occurs in Asia and South America (31% and 25%, respectively) (74). On a per capita basis, however, North America has the most freshwater available, at over 19,000 cubic meters per year, estimated in 1990. In contrast, the per capita amount is just over 4,700 cubic meters in Asia (including the Near East) (134).

By country, the amount of renewable freshwater available per capita on an annual basis ranges from over 600,000 cubic meters in Iceland to only 75 cubic meters per person in Kuwait, as estimated in 1995 (69).

There are also striking differences in water availability within countries. In Mexico,less than 10% of the land area provides more than half of the national rainwater runoff each year. Despite the fact that 90% of Mexico is arid and chronically water-short, its total per capita water availability in 1990 was over 4,000 cubic meters. Such a figure is grossly misleading as a measure of actual water availability for most Mexicans (30).

"The disparities between the rich and poor are never more stark than when it comes to access to water," according to the UN publication Earth Times (193). "Ask a person in New York what he thinks about the water problem and he will probably say, `what problem?' Ask a person in New Delhi and you will be lucky if you escape with a 15-minute lecture on how the water flows once a day, it has to be stored, it smells and, if you drink it without boiling it, chances are you will get sick" (193).

Throughout much of the developing world the freshwater supply comes in the form of seasonal rains. Such rains run off too quickly for efficient use, as during the monsoons in Asia (139). India, for instance, gets 90% of its rainfall during the summer monsoon season, which lasts from June to September. For the other eight months the country gets barely a drop of rain. As a result of the seasonal nature of the water supply, India and some other developing countries can make use of no more than 20% of their potentially available freshwater resources (132).

Altering natural supply systems. As water-short societies have done for centuries, many countries attempt to move water from where it occurs in nature to where people want it, and also to store water for future use. The Egyptians built thousands of canals and irrigation ditches to capture the Nile's waters in order to grow crops. In the first century AD Roman engineers brought water into Rome via huge aqueducts from as far away as 100 kilometers (21).

Worldwide there are some 40,000 dams higher than 15 meters, most of them built in the last 50 years (199). Although dams help insure a steady water supply, they often imperil aquatic ecosystems by disrupting flood cycles, blocking river channels, altering water flows in rivers, floodplains, deltas, and other natural wetlands, and imperiling plant and animal life (141).