Demand for water varies greatly in different countries, depending on population and the prevailing level and pattern of socio-economic development. Marked differences exist between developed and developing countries. For example, average per capita domestic use of water in the United States is more than 70 times that in Ghana. Worldwide water use increased dramatically from about 1,360km3 in 1950 to 4,130km3 in 1990, and is expected to reach about 5,190km3 by 2000. That is more than 50 per cent of all available fresh water.
Agriculture is the main drain on the water supply.
By 2025, as much as two thirds of the world population could be under stress conditions that will limit development, especially in poor societies. This represents a drop of 37 per cent per person since 1970 as a result of population growth. Water scarcity occurs when the amount of water withdrawn from lakes, rivers or ground water is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, bringing about increased competition among potential demands. Scarcities are likely to occur sooner in regions where the per capita availability of water is low to start with, and with high population growth. They become more serious if per capita demand is growing due to changes in consumption patterns.