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Glaciers and icecaps cover about 10% of the world's landmass. These are concentrated in Greenland and Antarctica and contain ~70% of the world's freshwater. Unfortunately, most of these resources are located far from human habitation and are not readily accessible for human use.

According to the United States Geological Survey (USGS), 96% of the world's frozen freshwater is at the South and North poles, with the remaining 4% spread over 550 000 km2 of glaciers and mountainous icecaps measuring about 180 000 km3 (UNEP, 1992; Untersteiner, 1975; WGMS, 1998, 2002).

Groundwater is by far the most abundant and readily available source of freshwater, followed by lakes, reservoirs, rivers and wetlands:

  • Groundwater represents over 90% of the world's readily available freshwater resource (Boswinkel, 2000). About 1.5 billion people depend upon groundwater for their drinking water supply (WRI, UNEP, UNDP, World Bank, 1998).
  • The amount of groundwater withdrawn annually is roughly estimated at ~600-700 km3, representing about 20% of global water withdrawals (WMO, 1997).
  • A comprehensive picture of the quantity of groundwater withdrawn and consumed annually around the world does not exist.
Most freshwater lakes are located at high altitudes, with nearly 50% of the world's lakes in Canada alone. Many lakes, especially those in arid regions, become salty through evaporation, which concentrates the inflowing salts. The Caspian Sea, the Dead Sea, and the Great Salt Lake are among the world's major salt lakes.

Reservoirs are artificial lakes, produced by constructing physical barriers across flowing rivers, which allow the water to pool and be used for various purposes. The volume of water stored in reservoirs worldwide is estimated at 4 286 km3 (Groombridge and Jenkins, 1998)

Wetlands include swamps, bogs, marshes, mires, lagoons and floodplains. The 10 largest wetlands in the world by area are: West Siberian Lowlands (780 000-1 000 000 km2), Amazon River (800 000 km2), Hudson Bay Lowlands (200 000-320 000 km2), Pantanal (140 000-200 000 km2), Upper Nile River (50 000-90 000 km2), Chari-Logone River (90 000 km2), Hudson Bay Lowlands in the South Pacific (69 000 km2), Congo River (40 000-80 000 km2), Upper Mackenzie River (60 000 km2), and North America prairie potholes (40 000 km2) (Pidwiny, 1999).

The total global area of wetlands is estimated at ~2 900 000 km2 (Groombridge and Jenkins, 1998). Most wetlands range in depth from 0-2 metres. Estimating the average depth of permanent wetlands at about one metre, the global volume of wetlands could range between 2 300 km3 and 2 900 km3.

   
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