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 .