Earth's hydrosphere is composed of many reservoirs, places where water is stored. Water is an overlapping component of each of earth's spheres - the biosphere, geosphere, and atmosphere all contain water. Earth's water is stored in clouds, oceans, ice caps & glaciers, lakes, rivers, living organisms, soil, and as groundwater.

The largest of earth's reservoirs is the ocean. In fact, 97% of all the water on our planet can be found in the oceans. Only 3% of our planet's water is freshwater - water that can be used by living things. Most of the planet's freshwater is trapped in glaciers, ice caps, or underground aquifers. Only a small amount of freshwater is found on Earth's surface as lakes and rivers, yet this is the source for most of the water people use every day.

How long a molecule of water will stay in one of these reservoirs is known as its residence time. Water will stay in some reservoirs for a long period of time; for example, groundwater and ice sheets typically have long residence times. Water does not typically spend a long time in the atmosphere before it condenses and falls back to the earth as precipitation, so this particular reservoir has a short residence time. Soil also has a short residence time because water is readily lost to evaporation or easily seeps into the ground.

The following page will show the global distribution of Earth's water and provide statistics for which reservoirs store most of our planet's water.