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  • Publicado : 2 de mayo de 2011
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Water is the most widespread substance to be found in the natural environment. Water exists in three states: liquid, solid and invisible vapour. It forms the oceans, seas, Lakes Rivers and theunderground; waters found in the top layers of the earth´s crust and solid cover. In de solid state, it existed as ice and snow cover in polar and alpine regions. A certain amount of water is contained inthe air as water vapour, water droplets and ice crystals, as well as in the biosphere. Huge amounts of water are bound up in the composition of the different minerals of the earth´s crust and core.
Toassess the total water storage on the earth´s reliably is a complicated problem because water is so very dynamic. It is in permanent motion, constantly changing from liquid to solid on gaseous phase,and back again. It is usual to estimate the quantity of water found in the so-called hydrosphere. This is all the free water existing in liquid, solid or gaseous state in the in the atmosphere, onthe earth´s surface and in the crust down to a depth of 2000 meters. Current estimates are that the earth´s hydrosphere contains a huge amount of water – about 1386 million cubic kilometers. However,97.5% of this amount are saline waters and only 2.5% is fresh water. The greater portion of this fresh water (68.7%) is in the form of ice and permanent snow cover in the Antarctic, the arctic, and inthe mountainous regions. Next, 29.9% exists as fresh groundwaters. Only 0.26% of the total amounts of fresh waters on the earth are concentrated in lakes, reservoirs and river system where they aremost easily accessible for our economic needs and absolutely vital for water ecosystems.
These are the values for natural, static, water storage in the hydrosphere. It is me amount of water containedsimultaneously, on average, over along period of time – in water bodies, aquifers, and the atmosphere. For shorter time intervals such as a single year, a couple of seasons, or a few months, the...
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