Map collection

Water is transported in different forms within the hydrological cycle or ‘water cycle’. Igor Shiklomanov and Peter H. Gleick (Water in Crisis: A Guide to the World’s Fresh Water Resources, Oxford University Press, New York, 1993) estimates that each year about 502,800 km³ of water evaporates over the oceans and seas, 90% of which (458,000 km³) returns directly to the oceans through precipitation, while the remainder (44 800 km³) falls over land.

With evapo-transpiration totalling about 74,200 km³, the total volume in the terrestrial hydrological cycle is about 119,000 km³. About 35% of this, or 44,800 km³, is returned to the oceans as run-off from rivers, groundwater and glaciers. A considerable portion of river flow and groundwater percolation never reaches the ocean, having evaporated in internal runoff areas or inland basins lacking an outlet to the ocean. However, some groundwater that bypasses the river systems reaches the oceans. Annually the hydrological cycle circulates nearly 577,000 km³ of water.