United Nations Environment Program

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)