BIOSPHERE CYCLE - HYDROLOGIC or WATER CYCLE

HYDROLOGIC OR WATERCYCLE

1.     The Hydrologic or Water cycle is probably the most important of all the natural cycles in the biosphere.

2.     More than 97% of water in the biosphere is found in the oceans, the remaining 3% is found on the continents and in the atmosphere.

3.     But more than 70% of this latter portion is locked in glaciers and icecaps.

4.   The water on which human depends so heavily – lakes, streams and ground water – accounts for less than 1% of the total supply and it is this water , which is currently being used and reused in many parts of the world.

5.  The Hydrologic cycle of the biosphere depends on the reciprocity of evaporation and precipitation.

6.     Liquid water on the earth goes into the atmosphere as vapour by evaporation and transpiration of plants.

7.     The vapour is returned to earth as rain or snow. Figure illustrates the complete hydrologic cycle.

8.     Most evaporation occurs over the oceans but some oceans lose more water by evaporation than they gain by precipitation.

9.   The difference is made up by runoff and seepage from the continents, over which there is more precipitation than evaporation.

10.    The continent lose more than 50% of the precipitation through evaporation and the remainder is temporarily stored in lakes and rivers or as ground water which is later discharged into oceans.

11.    The global cycle can be summerised as shown in table below :

	Oceans	Continents(km3/yr)	Whole Earth
Precipitation	3,24,000	99,000	4,23,000
Evaporation	3,61,000	62,000	4,23,000
Gain by inflow	+37,000	-37,000	0

12.  Each year, an estimated 4,23,000 km³ of water is evaporated, and the same quantity is precipitated over the whole surface of the earth. 

13.  The amount of water, which is temporarily stored and is later discharged into the oceans is 37,000 km³ and this is the amount which is potentially available for human needs.

Reference Books :

Environmental Pollution Control Engineering, C.S. Rao .