Water is the most abundant substance on earth, the principal constituent of all living organisms and a major force constantly shaping the surface of the earth. It is also a key factor in air-conditioning the earth for human existence and in influencing the progress of civilization. Water on earth exists in three faces in a space called the hydrosphere which extends about 15 km up into the atmosphere and about 1 km down into the lithosphere, the crust of the earth.
Estimating the total amount of water on the earth and in the various processes of hydrological cycle has been a topic of scientific exploration since the second half of the nineteenth century. However, quantitative data are scarce, particularly over the oceans, and so the amounts of water in the various components of the global hydrologic cycle are still not known precisely.
World Water Balance
Table on world water resources of rivers from different continents.
|La Plata||S. America|
Table 1 estimated quantities of water in various rivers in continents on the earth. The total quantity of water is estimated to be about 1386 M km3. About 96.5% of the water of the earth is contained in the ocean as saline water which amounts to 1338 M km3. If the earth were a uniform sphere, this quantity would be sufficient to cover it to a depth of about 2.6 km (1.6 miles). Of the remainder, 1.7% is in the polar ice, 1.7% in ground water and 0.1% in the surface and atmospheric water systems. The atmospheric water system contains only 12900 km3 of water, or less than one part in 100000 of all water of the earth.
Of the fresh water of the earth, which amounts to about 35 M km3, about two-thirds is polar ice and most of the remainder is groundwater going down to a depth of 200 to 600 m. Most ground water is saline below this depth. Only 0.006% of fresh water, which amounts to 2129 km3, is contained in rivers. Biological water fixed in the tissues of plants and animals makes up about 0.003% of all fresh water equivalent to half the volume contained in the rivers.
Less than 4% of total river flow of the world is used for irrigation and the rest flows down to oceans. The rivers of the world annually discharge about 44700 km3 of water into the oceans. This amounts to an annual average flow of 1.417 M m3/s. The world’s largest river, the Amazon, has an average discharge of 200000 m3/s, i.e. one-seventh of world’s annual average value. The Brahmaputra and the Ganges rivers flow into the Bay of Bengal with a mean annual discharge of 16000 m3/s and 15600 m3/s, respectively.
Although the total water resources of the earth remain essentially constant, the distribution of this water is continuously changing on continents, in regions and within local drainage basins.
As civilization progresses, human activities gradually encroach on the natural water environment, altering the dynamic equilibrium of the hydrologic cycle and initiating new processes and events. Encroachment at one stage can cause serious repercussions at another stage of the hydrologic cycle. For example, because of the burning of fossil fuels, the amount of CO2 in the atmosphere is increasing. This could result in a warming of the earth and have far-reaching effects on global hydrology.
Special Thanks To,
Dr. Md. Abdul Halim, B.Sc. Engg. (Civil), BUET; M.Sc. Engg. (WRE), BUET; Ph.D