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Section 10.1 The Earth's water

The hydrosphere represents the Earth's water inventory, which makes up about 0.03% of the total mass of the Earth. About 97% of this water is found in the oceans, and 3% in freshwater reservoirs. On average, about 0.001% of the Earth's water is present in the atmosphere.

Of the Earth's freshwater inventory (3% of the hydrosphere), most is present in glacial ice (70%) or groundwater (30%), with less than 1% found in surface lakes and rivers. These surface sources thus represent just 0.0003% of the Earth's total water inventory.

From an Earth systems perspective, it is convenient to divide the water inventory into reservoirs of water with fluxes of material passing between them. This is the basis of the water cycle. The residence time of water in a given reservoir is the size of the reservoir divided by the total outward flux. This provides an indication of how long water resides in each reservoir, or how often this inventory is recycled:

\begin{equation*} \textrm{residence time} = \frac{\textrm{reservoir}}{\textrm{outward flux}} \end{equation*}

Because the atmospheric water inventory is (relatively) small and rapidly exchanges material with the surface, the atmospheric residence time for water is just 11 days. FOr comparison, the residence time in the ocean is about 3000 years