An important source of fresh water for terrestrial life is precipitation. Solar radiations vaporize large quantities of water both from land and oceans. The moisture condenses in the atmosphere and forms rains, dew or snow which is brought back to earth’s surface. It provides information about the quantities of fresh water which evaporates from earth’s surface, the total amount of precipitation and annual surface runoffs etc.
Annual water budget of our planet.
Water in cubic kms.
It will be noticed from Table 6.2 that oceans contribute about 4, 52,600 cubic kms of water annually to atmosphere but receive only 4, 11,600 cubic kms as precipitation. The deficit is balanced by 41,000 cubic kms of surface and sub-surface runoff which they receive. On the other hand land surface contributes about 72,500 cubic kms of water to the atmosphere while it receives 1, 13,500 cubic kms of water as precipitation.
The excess amount is drained off as surface and ground water runoff to oceans. So the amount of water present in sea, on land surface, underground water, water present in atmosphere as vapours etc. are in a state of dynamic equilibrium. The excess water received by land surface, about 41,000 cubic kms has to flow back to sea – it cannot be retained on earth’s crust ordinarily.
The total annual precipitation of 5, 25,100 cubic kms is not evenly distributed over earth’s surface. At a given point of time, the amount of perceptible moisture present in the atmosphere is maximum at equator being equivalent to about 44 mm of rains. At latitude of 40°-50° North and South, the available perceptible moisture would be about 25 mm during summers and 10 mm during winters as rain equivalent.
At poles this yield ranges from 2 mm in winters and 8 mm in summers as rain equivalent. The amount of perceptible moisture in atmosphere is subject to large variations which depend on a number of factors. However, it does make the equatorial belt the wettest zone. Rainfall decreases as we move out on either side of equator acquiring a seasonal character.