On the earth, 361 million sq. km area of the whole earth is completely covered with water.
While the remaining 149 million sq. km ground area also has water in form of rivers, glaciers, lakes, marshy land and groundwater.
The hydrological cycle also plays an important role in distribution of water on earth.
After evaporation, water from oceans, seas, lakes and rivers and transpiration from plants reaches the atmosphere and again converts into water and reaches the ground. Temperature plays an important role in changing the form of water. Hydrological cycle maintains the water balance on the earth.
In nature, only 2 per cent of the total water on earth is in the form of fresh water, out of which two-third is in the form of snow caps and glaciers on poles. Of the remaining quantity of water, a little less than one per cent is directly available for living organisms.
If the total ice of the earth melts, all the rivers of the earth can flow at the current speed for the coming 1,000 years, but this cannot happen all of a sudden. It is a long-term process of climatic change in nature whose balanced implementation depends on many geographical factors.
Water remains stable at different places for different periods. Stability of water in plants and organisms remains stable up to one week. In the atmosphere it remains stable for 8 to 10 days, in rivers for two weeks, in soil from two weeks to one year, in lakes and humid soils for years and in oceans for thousands of years.
In the system of hydrological cycle, distribution of water goes on changing with time. Such changes occur at different rates in different environmental systems. During the Snow Age, the maximum part of water was frozen in the form of snow, due to which the water level of seas had gone down. After a certain period, on completion of Snow Age, the water in seas rose high due to the meaning of snow cover. In the present times also, due to rise in temperature, the snow cover is constantly reducing resulting in expansion of seas.
Due to rise in world temperature, the snow of Antarctica and Greenland is melting. Snow of the highest and most important mountain peak Kilimanjaro is melting fast in Africa. About 90 per cent of world’s fresh water is found in a vast area of 1, 40, 00,000 sq. km of Antarctica. This plays a dominant role in regulating the marine ecological system and climate of the world.
Thus, the present water distribution in nature is also not of permanent nature but has been changing. Three decades ago, there was sufficient water available but a water crisis is being faced today. On the other side, excess water is also a problem at some other places. Every year 40,000 cubic kl. water flows from the ground towards the seas, out of which man can use only 9,000 cubic kl. water. Similarly, about 5 lakh cubic kl. water evaporates from the seas and goes into the atmosphere. It is clear that though water is distributed over a vast area but still it is only partially available for mankind.