HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE

HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE
The cyclic movement of water between the atmo­sphere, the land and the sea is called the water cycle. It involves the movement of water in a great series of continuous interchanges 'of both geographical position and physical change. Water is released into the atmosphere as water vapour through evaporation from the oceans, rivers, and lakes, and through evapo-transpiration from plants and the ground surface. Oceans are the biggest reservoirs of free water, from which the evaporation totals about 455,000 cu km per year. Evaporation from soil, plants and water surfaces of the continents totals 62,000 cu km per year.

Within the atmosphere water vapour condenses to form clouds and is returned'to the land and to its water bodies as precipitation. This quantity of precipitation must be equal to the quantity of evaporation that takes place. Precipitation is unevenly distributed between lan~ and oceans. The amount of precipitation gained by the land is more than the evaporation that takes place from the land surface.

This excess quantity flows over or under the ground surface to reach the sea; collectively called runoff, this forms a continuous cycle. However, great inequalities exist in the global amounts of water stored in gaseous, liquid and solid states.

Water flowing exposed or ponded upon land is surface water; water occupying openings in the soil, overburden or bedrock is subsurface water.