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8.1: Distribution of Water on Earth

Difficulty Level: At Grade Created by: CK-12
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Water, water everywhere. But how much of it is useful?

Earth is the water planet. From space, Earth is a blue ball, unlike any of the other planets in our solar system. Life, also unique to Earth of the planets in our solar system, depends on this water. While there's a lot of salt water, a surprisingly small amount of it is fresh water.

Distribution of Water

Earth’s oceans contain 97% of the planet’s water. That leaves just 3% as fresh water, water with low concentrations of salts (Figure below). Most fresh water is trapped as ice in the vast glaciers and ice sheets of Greenland and Antarctica.

How is the 3% of fresh water divided into different reservoirs? How much of that water is useful for living creatures? How much for people?

Diagram of the distribution of Earth's water

The distribution of Earth’s water.

A storage location for water such as an ocean, glacier, pond, or even the atmosphere is known as a reservoir. A water molecule may pass through a reservoir very quickly or may remain for much longer. The amount of time a molecule stays in a reservoir is known as its residence time.


  • Of Earth's water, 97% is in the oceans.
  • Of the remaining 3%, much is trapped in ice and glaciers.
  • A substance is stored in a reservoir and the amount of time it stays in that reservoir is its residence time.


  1. If Earth is the water planet, why is water sometimes a scarce resource?
  2. What are the reservoirs for water?
  3. In which reservoirs does water have the longest residence times? The shortest?

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    A storage location for a substance, such as water. The atmosphere is a reservoir for carbon dioxide.

    residence time

    The amount of time, on average, a substance remains in a reservoir.

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    Difficulty Level:
    At Grade
    Date Created:
    Feb 24, 2012
    Last Modified:
    Aug 07, 2016
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