China and India, the world's first and second most populous countries, provide examples of how even modest population growth rates translate into large absolute numbers when the population base is large. In China the population growth rate is about 1% per year, estimated in 1998 (135). Nevertheless, because China's population is over 1.2 billion people, even a low population growth rate means 12 million additional people each year. India's population growth rate, which is substantially higher than China's, at about 1.9% per year, means about 18 million people added each year to India's current population of about 970 million (180).

In the two regions of the world that already face the most serious absolute or seasonal shortages of water—Africa and the Near East—population growth rates remain among the highest in the world. In sub-Saharan Africa population is growing by an average 2.6% a year; in the Near East and North Africa, by 2.2% (135). These population growth rates have ominous implications for per capita water supply in the countries of these regions (see Table 1). Water stress and scarcity. As their populations grow, more and more countries are facing water shortages (62). A country is said to experience water stress when annual water supplies drop below 1,700 cubic meters per person. At levels between 1,700 and 1,000 cubic meters per person, periodic or limited water shortages can be expected. When annual water supplies drop below 1,000 cubic meters per person, the country faces water scarcity (57, 69, 139). Once a country experiences water scarcity, it can expect chronic shortages of freshwater that threaten food production, hinder economic development, and damage ecosystems.

Malin Falkenmark developed the concepts of water stress and water scarcity based on an index of per capita freshwater needs. She estimated a minimum need of 100 liters per day per person for household use and from 5 to 20 times as much for agricultural and industrial uses (65, 69). These concepts have been widely accepted and used by hydrologists, the World Bank, and other organizations. For example, Population Action International (PAI) has relied on them to make projections of per capita water availability and to forecast water shortages in 2025 and 2050 (49, 69) (see Table 1).

Calculations of water stress and water scarcity are based on estimates of a country's renewable freshwater supplies and do not include water withdrawn from fossil groundwater (see glossary). Fossil groundwater is essentially a nonrenewable resource: It takes tens of thousands of years for these deep aquifers to replenish themselves. A country may temporarily avoid the effects of water stress by mining its nonrenewable water supplies, but this practice is not sustainable, particularly if the population continues to grow rapidly and per capita demand for freshwater increases.

As of 1995, 31 countries, with a combined population of over 458 million, faced either water stress or water scarcity (48) (see Figure 5). This represents an addition of only 3 countries since 1990, when 28 countries, with a combined population of 335 million faced chronic water stress or water scarcity (49). The number of people estimated to live in water-scarce and water-stressed countries rose by nearly 125 million during these five years, however, largely reflecting population growth in water-short countries.