<P> "Every year the turnover of water on Earth involves 577,000 km of water . This is water that evaporates from the oceanic surface (502,800 km) and from land (74,200 km). The same amount of water falls as atmospheric precipitation, 458,000 km on the ocean and 119,000 km on land . The difference between precipitation and evaporation from the land surface (119,000 - 74,200 = 44,800 km / year) represents the total runoff of the Earth's rivers (42,700 km / year) and direct groundwater runoff to the ocean (2100 km / year). These are the principal sources of fresh water to support life necessities and man's economic activities ." </P> <P> Water is a basic necessity of life . Since 2 / 3 of the Earth is covered by water, the Earth is also called the blue planet and the watery planet . Hydrosphere plays an important role in the existence of the atmosphere in its present form . Oceans are important in this regard . When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of Mercury . Later the gases hydrogen and helium were expelled from the atmosphere . The gases and water vapor released as the Earth cooled became its present atmosphere . Other gases and water vapor released by volcanoes also entered the atmosphere . As the Earth cooled the water vapor in the atmosphere condensed and fell as rain . The atmosphere cooled further as atmospheric carbon dioxide dissolved in to rain water . In turn this further caused the water vapor to condense and fall as rain . This rain water filled the depressions on the Earth's surface and formed the oceans . It is estimated that this occurred about 4000 million years ago . The first life forms began in the oceans . These organisms did not breathe oxygen . Later, when cyanobacteria evolved, the process of conversion of carbon dioxide into food and oxygen began . As a result, Earth's atmosphere has a distinctly different composition from that of other planets and allowed for life to evolve on Earth . </P> <P> According to Igor A. Shiklomanov, it takes 2500 years for the complete recharge and replenishment of oceanic waters, 10,000 years for permafrost and ice, 1500 years for deep groundwater and mountainous glaciers, 17 years in lakes and 16 days in rivers . </P> <P> "Specific water availability is the residual (after use) per capita quantity of fresh water ." Fresh water resources are unevenly distributed in terms of space and time and can go from floods to water shortages within months in the same area . In 1998 76% of the total population had a specific water availability of less than 5.0 thousand m3 per year per capita . Already by 1998, 35% of the global population suffered "very low or catastrophically low water supplies" and Shiklomanov predicted that the situation would deteriorate in the twenty - first century with "most of the Earth's population will be living under the conditions of low or catastrophically low water supply" by 2025 . There is only 2.5% of fresh water in the hydrosphere and only 0.25% of water is accessible for our use . </P>

Approximately what percentage of the earth's surface is covered by water