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[[File:Earth's_water_distribution.svg|thumb|upright=1.2|A graphical distribution of the locations of water on Earth.{{efn|Only 3% of the Earth's water is fresh water. Most of it is in icecaps and glaciers (69%) and groundwater (30%), while all lakes, rivers and swamps combined only account for a small fraction (0.3%) of the Earth's total freshwater reserves.{{citation needed|date=November 2022}}}}]]
{{main|Water distribution on Earth}}
Saline water in [[ocean]]s, [[sea]]s and saline [[groundwater]] make up about 97% of all the water on [[Earth]]. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in [[glacier]]s, [[ice]] and snow, 0.5–0.75% as fresh groundwater. The water table is the level below which all spaces are filled with water, while the area above this level, where spaces in the rock and soil contain both air and water, is known as the unsaturated zone. The water in this unsaturated zone is referred to as soil moisture.
Below the water table, the entire region is known as the saturated zone, and the water in this zone is called groundwater.<ref name=":0">{{Citation |title=Natural Quality of Water and Groundwater Contamination |date=2000-04-14 |url=http://dx.doi.org/10.1201/9781482278934-9 |work=Groundwater Contamination, Volume I |pages=35–56 |access-date=2023-12-19 |publisher=CRC Press|doi=10.1201/9781482278934-9 |isbn=978-0-429-18165-8 }}</ref> Despite being a hidden resource whose value is often overlooked,Groundwater plays a crucial role as the primary source of water for various purposes including drinking, washing, farming, and manufacturing, and even when not directly used as a drinking water supply, it remains vital to protect due to its ability to carry contaminants and pollutants from the land into lakes and rivers, which constitute a significant percentage of other people's freshwater supply,. and groundwaterIt is almost ubiquitous underground, residing in the spaces between particles of rock and soil or within crevices and cracks in rock, typically within {{cvt|100 metres|m|ft}} of the surface,<ref name=":0" /> and [[soil]] moisture, and less than 0.01% of it as [[surface water]] in [[lake]]s, [[swamp]]s and [[river]]s.<ref>[http://ga.water.usgs.gov/edu/earthwherewater.html Where is Earth's water?] {{webarchive|url=https://web.archive.org/web/20131214091601/http://ga.water.usgs.gov/edu/earthwherewater.html |date=14 December 2013 }}, [[United States Geological Survey]].</ref><ref>[http://www.physicalgeography.net/fundamentals/8b.html Physicalgeography.net] {{webarchive|url=https://web.archive.org/web/20160126072955/http://www.physicalgeography.net/fundamentals/8b.html |date=26 January 2016 }}. Physicalgeography.net. Retrieved on 29 December 2012.</ref>
Freshwater lakes contain about 87% of this fresh surface water, including 29% in the [[African Great Lakes]], 22% in [[Lake Baikal]] in Russia, 21% in the [[North American Great Lakes]], and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably the [[Amazon River]]. The atmosphere contains 0.04% water.<ref>{{cite book |last= Gleick |first= Peter |author-link=Peter Gleick|editor= Stephen H. Schneider |title= Encyclopedia of Climate and Weather|url= https://archive.org/details/encyclopediaofcl02schn |url-access= registration |publisher= Oxford University Press |year= 1996|display-authors=etal}}</ref> In areas with no fresh water on the ground surface, fresh water derived from [[precipitation (meteorology)|precipitation]] may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in [[ice sheet]]s. Many areas have very little fresh water, such as [[desert]]s.
== Freshwater ecosystems ==
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