The water cycle consists of precipitation, vapour transport, evaporation, evapo-transpiration, infiltration, groundwater flow and runoff. Figure 1 explains the global water cycle, illustrating how nearly 577,000 km3 of water circulates through the cycle each year. A table of estimated residence times shows the approximate times that water resources exist as biospheric water, atmospheric water and so on.
The world’s surface water is affected by varying levels of precipitation, evaporation and runoff, in different regions. Figure 2 illustrates the different rates at which these processes affect the major regions of the world, and the resulting uneven distribution of freshwater. Water is transported in various forms within the hydrologic cycle. Shiklomanov in Gleick (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 and evaporation totalling about 74,200 km3, the total volume in the terrestrial hydrologic cycle is about 119,000 km3. Around 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 which lack outlets to the ocean. However, some groundwater that bypasses the river systems reaches the oceans. Annually the hydrologic cycle circulates nearly 577,000 km3 of water (Gleick, 1993).