- Annually, around 505,000km3 is evaporated from the oceans, and 72,000km3/year is evaporated from land surfaces to join the hydrological cycle. This gives a total of around 577,000km3/year active in the global water cycle. Of this total, approximately 458,000km3/year (80%) falls back onto the oceans and only 20% (119,000km3/year) falls onto the land.
- Of the 20% freshwater falling on land as precipitation, most is transpired back into the atmosphere almost immediately, leaving only 8% of the total active volume on the ground. Much of this forms groundwater that may become inaccessible and/or polluted and surface water which may become polluted.
- Globally, 7,000km3 more water is stored on land in March than in September when 600km3 more is stored in the atmosphere than in March.
Source: Gleick, Peter (2001) “The World’s Water: The Biennial Report on Freshwater Resources” Island Press, p22
Atmospheric water volume taken from: Ball, Philip (1999) “H2O: A Biography of Water” Pheonix/Orion, London.
A good explanation of the hydrological cycle can be found at The UK Groundwater Forum
