The "water cycle" is the name given to the constant cycle of evaporation and percipitation in the Eearth's atmosphere.
Solar radiation powers the evaporation part of the world's water cycle. The water of the world's oceans covers about seventy-one per cent of the planet's surface and about eighty-six per cent of evaporation on the planet takes place of the oceans and only fourteen per cent from the land which covers almost thirty per cent of the planet.
Of the water which is evaporated into the Earth's atmosphere, only 79 per cent is returned to the oceans as percipitation. Twenty-one per cent of the water falls as percipitation on the land which produced 14 per cent of the evaporation. The seven per cent excess returns to the oceans by runoff, the flow of water through streams and rivers.
Globally, runoff occurs because of the imbalance between evaporation and precipitation over the Earth's land and ocean surfaces.
The runoff is not equal over the world's land surface but is greatest in those regions which recieve high rates of precipitation but have only low rates of evaporation;-
| Continent | Runoff Per Unit Area
in mm per year |
| Europe |  | 300 |
| Asia | | 286 |
| Africa | | 139 |
| South America | | 445 |
Australasia
(Australia, New Zealand & New Guinea) | | 218 |
| Antarctica and Greenland | | 164 |
| - ML Lvovitch, 1972, World Water Balance, Symposium of World Water Balance, IASH-UNESCO, Report Number 92 |
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