According to the UNEP (United Nations Environment Program, Global Environment Outlook 3, 2002), the total volume of water on earth is about 1,400 million cubic km (kilometers), of which, only 2.5 per cent, or about 35 million cubic km, is freshwater. Most freshwater is locked up in glaciers, or in deep groundwater aquifers. The usable portion of freshwater is only about 200,000 cubic km of water. This is less than 1 per cent of all freshwater and, only 0.01 per cent of all water on earth.
Due to the high population increase, rapid industrialization and expansion of irrigated agriculture, there has been increased use of freshwater. About one-third of the world’s population lives in countries suffering from moderate-to-high water stress, where water consumption is more than 10 per cent of renewable freshwater resources. It is estimated that, in less than 25 years, two-thirds of the world’s people will be living in water-stressed countries (UNEP, 2002).