The main stores in the hydrological cycle are the oceans, glaciers and ice caps, and groundwater. Oceans contain approximately of Earth's water, while glaciers and ice caps contain and groundwater contains another . This is core content for IB ESS SL and HL, Subtopic 4.1 Water systems.
How Water Is Stored
A storage is a location where water is held within the hydrological system. Water moves between storages through flows, including evaporation, precipitation, infiltration, surface run-off and groundwater flow; these movements are driven mainly by solar radiation and gravity.
| Water storage | Approximate percentage of Earth's water | Significance |
|---|---|---|
| Oceans | The largest store; mainly saline water and the major source of evaporation | |
| Glaciers and ice caps | Frozen freshwater with potentially long residence times | |
| Groundwater | Water stored below ground in soil and permeable rocks, including aquifers | |
| Surface freshwater | Lakes, rivers and other accessible surface stores | |
| Atmosphere |
The three largest stores together contain approximately:
Therefore, only a very small proportion of Earth's water is stored as surface freshwater, in the atmosphere or in organisms. A common misconception is that most freshwater is readily available: much of it is frozen or stored underground, and accessibility depends on location, depth, quality, technology and economic conditions. This connects the physical hydrological system to water security and environmental justice.
Exam Technique
For a state question, give the three stores and their percentages. For an explain question, distinguish storages from flows and describe how water moves between them. In a systems diagram, represent storages as rectangular boxes and flows as labelled arrows; do not incorrectly label evaporation or precipitation as stores.