Estimates have been made of the quantities of water that are stored in various global compartments. By far, the largest quantity of water occurs in the deep lithosphere, which contains an estimated 27×1018 tons (27-billion-billion tons) of water, or 94.7% of the global total. The next largest compartment is the oceans, which contain 1.5×1018 tons, or 5.2% of the total. Ice caps contain 0.019×1018 tons, equivalent to most of the remaining 0.1% of Earth's water. Although present in relatively small quantities compared to the above, water in other compartments is very important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7×1014 tons), inland surface waters such as lakes and rivers (0.27×1014 ton), and the atmosphere (0.14×1014 tons).
The smallest compartments of water also tend to have the shortest turnover times, because their inputs and outputs are relatively large in comparison with the mass of water that is contained. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8×1014 tons as evaporation from the oceans (4.1×1014 tons/yr) and terrestrial ecosystems (0.65×1014 tons/yr), and turns over about 34 times per year.
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