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Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. About 1.460 petatonnes (Pt) (1021 kilograms) of water covers 71% of the Earth's surface, 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 (formed of solid and liquid water particles suspended in air), and precipitation.

Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. Some of the Earth's water is contained within water towers, biological bodies, manufactured products, and food stores. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land.

Water moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year to the precipitation of 107 Tt per year over land. Clean, fresh drinking water is essential to human and other life. However, in many parts of the world — especially developing countries — there is a water crisis, and it is estimated that by 2025 more than half of the world population will be facing water-based vulnerability. Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of freshwater is consumed by agriculture.

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The water cycle — technically known as the hydrologic cycle — is the circulation of water within the Earth's hydrosphere, involving changes in the physical state of water between liquid, solid, and gas phases. The hydrologic cycle is the continuous exchange of water between atmosphere, land, surface and subsurface waters, and organisms. In addition to storage in various compartments (the ocean is one such "compartment"), the multiple cycles that make up the earth's water cycle involve five main physical actions: evaporation, precipitation, infiltration, runoff, and subsurface flow.

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Did you know...

DYK Question Mark

... that the International Water Management Institute celebrated its 25th anniversary last year?

... that the International Water Centre celebrated its 5th birthday last year?

... that the National Rural Water Association represents more than 26,242 water and wastewater utility members?

... that "water bears" are small, segmented animals that can survive in a dehydrated state for nearly 10 years?

... that there are at least 15 different forms of crystalline ice?

... that water memory is a controversial homeopathic concept, which holds that water is capable of containing "memory" of particles dissolved in it?

... that there are more particles in a glass of water than grains of sand on earth?

... that there is a new definition for Earth's water cycle that includes the three "interactive" cycles: cosmic water cycle; atmospheric water cycle, and oceanic water cycle (new ocean recycled from center of Earth about every 7 million years) The Water Channel TV

... that Agroforestry and hedges are solutions to build micro-climates and allow the circulation of water to inland thanks to the phenomena of evapotranspiration of plants. For example, one hectare of beech forest, which consumes 2,000 to 5,000 tonnes of water per year, yields 2,000 tonnes per evaporation (https://www.onf.fr/onf/forets-et-espaces-naturels/+/1f::comprendre-la-foret.html french translation)

Water Rights

  • "The Arizona Court of Appeals recently upheld a decision that the Arizona Department of Water Resources has the right to issue instream flow permits and affirmed the principle that instream flow and in situ water rights need not be diversionary." [1]

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  • WikiProject Lakes describes the Earth's lakes. The project aims to consolidate and unify pages relating to lakes around the world.

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