Water stores and cycles

Water is stored in three states: as liquid water in the oceans, river and lakes; as ice; and as atmospheric moisture in the form of gaseous water vapour. Water is cycled between key stores by a variety of processes at different rates of flux.

Liquid water storage (Hydrosphere): This storage accounts for 96.5% of all water on earth. Processes impacting upon this important store include runoff and precipitation inputting water to the store and evaporation moving water from the ocean into the atmospheric store. These changes have minimal impact upon the storage capacity, however long term climatic change events such as ice ages do have the potential to lower the storage capacity significantly.

Ice storage (Cryosphere): This storage accounts for 1.7% of all water on earth. Processes impacting upon the store include precipitation (as snow) and outputs include ice melt. Major stores include the Antarctic and Greenland ice sheets, polar sea ice and mountain glaciers. Annual changes to ice coverage have minimal impact upon storage capacity. However, as with the hydrospheric storage, during ice ages cryospheric storage increases and during warmer inter-glacials it reduces.

Bedrock storage (Lithosphere): This storage also accounts for 1.7% of all water on earth. Whilst the level of storage capacity is low, this store captures water for the longest periods of time. Water can flow through the lithosphere into underground aquifers but this transfer may be relatively slow, often taking many years. Some water is stored within bedding planes, joints and pores in rocks and can remain there for hundreds of years.

Atmospheric moisture storage (Atmosphere): This storage accounts for 0.001% of water on earth. Water is removed from water surfaces through evaporation and is then stored temporarily as water vapour and condensation before being released back to earth as precipitation. Additionally, transpiration from plants releases water vapour into the atmosphere.

These four dominant ‘spheres’ are supplemented by two others that have a role: the Biosphere (all living things, including plants and animals) and the Pedosphere (the soil layer that contains both organic carbon as soil bacteria and remains of plants, and non-organic carbon from infiltration by acidic rain).