GEO: Global Environment Outlook

Global overview

The total volume of water on Earth is about 1 400 million km³ of which only 2.5 per cent, or about 35 million km³, is freshwater (see table below). Most freshwater occurs in the form of permanent ice or snow, locked up in Antarctica and Greenland, or in deep groundwater aquifers. The principal sources of water for human use are lakes, rivers, soil moisture and relatively shallow groundwater basins. The usable portion of these sources is only about 200 000 km³ of water — less than 1 per cent of all freshwater and only 0.01 per cent of all water on Earth. Much of this available water is located far from human populations, further complicating issues of water use.

The replenishment of freshwater depends on evaporation from the surface of the oceans. About 505 000 km³, or a layer 1.4 metres thick, evaporates from the oceans annually. Another 72 000 km³ evaporates from the land. About 80 per cent of all precipitation, or about 458 000 km³/year, falls on the oceans and the remaining 119 000 km³/year on land. The difference between precipitation on land surfaces and evaporation from those surfaces (119 000 km³ minus 72 000 km³ annually) is run-off and groundwater recharge — approximately 47 000 km³ annually (Gleick 1993). The figure opposite shows one estimate of the average annual water balance of major continental areas, including precipitation, evaporation and run-off. More than one-half of all run-off occurs in Asia and South America, and a large fraction occurs in a single river, the Amazon, which carries more than 6 000 km³ of water a year (Shiklomanov 1999).

Freshwater: Asia and the Pacific

The Asia and the Pacific Region accounts for about 36 per cent of global run-off. Even so, water scarcity and pollution are key issues and the region has the lowest per capita availability of freshwater: renewable water resources amounted to about 3 690 m³ per capita/year in mid-1999 for the 30 largest countries in the region for which records are available (UNDP, UNEP, World Bank and WRI 2000 and United Nations Population Division 2001).

In absolute terms, China, India and Indonesia have the largest water resources, more than one-half of the region’s total. Several countries, including Bangladesh, India, Pakistan and the Republic of Korea, already suffer from water scarcity or water stress. More will do so as populations and consumption increase. Agriculture is the biggest consumer (86 per cent), with smaller amounts going to industry (8 per cent) and domestic use (6 per cent) (compiled from UNDP, UNEP, World Bank and WRI 2000).