Water is a chemical substance, composed of hydrogen and oxygen, vital for all known forms of life.[1] In typical usage, water refers only to its liquid state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Covering 71% of the Earth's surface,[2] it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds, and precipitation.[3]

Water on Earth moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, andrunoff, usually reaching the sea. Over land, evaporation and transpiration contribute to the precipitation over land.

Clean, fresh drinking water is essential to human and other lifeforms. However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.[4] A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.[5]

This outline is provided as an overview of and topical guide to water:

Chemical properties and use

Physical properties



In nature and life

Marine life

Politics and issues

Supply and sanitation

In culture and sport




  1. United Nations
  2. "CIA- The world fact book". Central Intelligence Agency. Retrieved 2008-12-20.
  3. Water Vapor in the Climate System, Special Report, [AGU], December 1995 (linked 4/2007).Vital Water UNEP.
  4. Kulshreshtha, S.N (1998). "A Global Outlook for Water Resources to the Year 2025". Water Resources Management 12 (3): 167–184. doi:10.1023/A:1007957229865.
  5. "Charting Our Water Future: Economic frameworks to inform decision-making"

See also

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