The Earth is not a planet as any other of the solar system. That we know, is the only one on which life has developed. The rationale for this originality is its position relative to the Sun. This position determines precisely the climate of our planet, a climate which has been conducive to the emergence of life. But such a climate is the result of a balance constantly readjusted among the water, the atmosphere and the energy of the Sun.

Solar energy, which allows you to keep moving between the Earth, the sea and the atmosphere, bodies of water makes the water cycle engine. We thus see appear the close link between the Earth’s climate and the water cycle. On Earth, water is present in all its forms: liquid, ice or steam. It is distributed in five interconnected reserves whose Assembly constitutes the hydrosphere. The sea is the most important of these reserves, followed by deposits of ice or snow, terrestrial waters, the atmosphere and finally the biosphere. More precisely, the Sea contains 1350 x 1015 m3 of water, i.e., 97% of all the water in the hydrosphere. The continents have 33.6 x 1015 m3, mainly located in the Arctic and Antarctic ice. The atmosphere contains a hundredthousandth fraction of the water content of the climate system: 0,013 x 1015 m3.

It is possible that the examination of this figure provokes a certain surprise. The abundance of precipitation, as well as their influence on climate and hydrological resources, does not seem commensurate with the minimum amount of water present in the atmosphere at a given time. In the continents, water is distributed among different reservations, the most important of which, and from afar, is formed by glaciers (25 x 1015 m3) totaling about 1.8 per cent of the hydrosphere; they come below the waters surface (8.4 x 1015 m3), lakes and rivers (0.2 x 1015 m3) Finally the living matter of the biosphere (0.0006 x 1015 m3. In deposits formed by underground waters, waters that steep soils (tile) cannot represent more than 0.066 x l0l5 m3.The rest is distributed almost evenly among deposits of less than or equal to 800 m depth. These deposits are subject to incessant transfers of large amounts of water within the climate system. We all have an intuitive notion of the cycle of water, at least in its broad lines: evaporation, precipitation, water courses. Everything comes from the sea and everything returns to the sea in one way or another. The residence time of water in each type of reservation can be calculated from the amount of water present in the reserve and its rate of accumulation or disappearance. In the atmosphere, the dwell time is about nine days; in polar ice and sea, on the other hand, is several thousands of years.