Water Cycle

updated February 2017

Our planet is a miserly distributor of freshwater. Most water is rendered useless to humanity by dilution with salt in the ocean. Only 2.5% is available as freshwater, of which two thirds is locked up in ice and snow.

The water cycle is driven by evaporation from land and sea, condensing into clouds which have the potential for precipitation as rain. Again, nature is unkind in depositing almost 80% of rain over the sea.

Of the rain that falls over land, only 40% finds its way as “blue water” into aquifers, lakes and rivers which are accessible supply sources. The “green water” balance is absorbed by the land, of great potential value to agriculture but notoriously fickle for that purpose in volume, timing, intensity and location.

Thanks to this natural cycle, the movement and action of water is a renewable source of energy and life. However, unlike other renewable resources such as sun, wind and tide, freshwater availability is not plentiful. And the amount of water on the planet is fixed, on the human timescale.

The advent of human civilization has superimposed its own version of the water cycle on nature. We withdraw water for our various contemporary uses and discharge whatever remains back into the natural cycle.

The most obvious interference with the natural water cycle is the construction of a major dam, or other deliberate disturbance of the natural course. Examples include the Inga 3 mega-project in the Democratic Republic of Congo and India’s controversial river-linking project to transfer freshwater from the north to south of the country.

Quite apart from inevitable human displacement, altering the natural flow of a river can destroy sensitive delta ecosystems on which freshwater fishing and agriculture depend. There are 48,000 large dams in place around the world, with a further 3,700 either planned or under construction.

Similar unnatural degradation is found in aquifers, underground reserves charged by the passage of water through soil and rocks. If withdrawals exceed the natural rate of recharge, the level of an aquifer will fall, eventually drying up altogether. In parts of India, the water table is believed to have fallen more than 300 metres. Outdated laws permitting landowners to indulge in unlimited and free access to water apparently remain beyond the reach of government action.

Many cities are located above these aquifers and are known to be sinking. This factor was one cause of the serious flooding in Bangkok towards the end of 2011. In coastal regions, depleted aquifers increase the risk of saline intrusion.

Water discharged from the anthropogenic cycle is rarely in its original condition. For example. agro-chemical pollution through run-off of nitrates and phosphates causes eutrophication, the excessive growth of algae whose eventual decomposition removes oxygen from the water, killing the aquatic ecosystem.

“Freshwater use” is listed as one of nine planetary boundaries in the influential studies published by the Stockholm Resilience Centre. These endorse the assertions of hydrologists that, at global level, there is enough water to meet human needs. At regional level the picture is very different, with damage to some ecosystems approaching “tipping points” which can trigger abrupt change.

Lake Chad is one example. Misguided governance of the natural cycle of the Lake led to its area of water collapsing by 90% in the space of 30 years, affecting 20 million people.

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The Water Cycle explained by the National Science Foundation


World dams since 1800 from CSDMSmovie


Jonathan Foley, Director of the Institute on the Environment, University of Minnesota
explaining why we should be more concerned about potential tipping points in the earth’s ecosystem.

more Water Scarcity briefings
Definitions
Water Energy Food Nexus
Causes of Water Scarcity
Climate Change and Water Scarcity
Solutions to Water Scarcity
Sustainable Development Goal for Water
Access to Drinking Water
Water Wars
Source material and useful links

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