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Distribution of Water on Earth

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Distribution of Water on Earth

Where is Earth's Water?

Here's a photo of Earth from space again. It's pretty easy to see where most of Earth's water is. It's in the oceans. Nearly all of Earth's water is in the seas. Although it's extremely useful for ocean ecosystems, it's not very useful for people. So where is the water that we can use?

Where Is Earth’s Freshwater?

Most of Earth’s water is salt water in the oceans. As seen below, only 3 percent of Earth’s water is fresh ( Figure below ). Freshwater is water that contains little or no dissolved salt. Most freshwater is frozen in ice caps and glaciers. Glaciers cover the peaks of some tall mountains. For example, the Cascades Mountains in North America and the Alps Mountains in Europe are capped with ice. Ice caps cover vast areas of Antarctica and Greenland. Chunks of ice frequently break off ice caps. They form icebergs that float in the oceans.

Diagram of the distribution of Earth's water

What percentage of Earth’s surface freshwater is water vapor in the air?

Only a tiny fraction of Earth’s freshwater is in the liquid state. Most liquid freshwater is under the ground in layers of rock. Of freshwater on the surface, the majority occurs in lakes and soil. What percentage of freshwater on the surface is found in living things?

Water can remain in a particular location for a long time. This is known as a reservoir . Reservoirs for water include oceans, glaciers, ponds, or even the atmosphere. 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 . The residence time of water in the ocean is 3,200 years. Of course, not every molecule stays that long. That number is an average for all molecules.

Vocabulary

  • freshwater : Water that contains little or no dissolved salt.
  • reservoir : Storage location for a substance, such as water. The atmosphere is a reservoir for carbon dioxide.
  • residence time : Amount of time, on average, a substance remains in a reservoir.

Summary

  • 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. The amount of time it stays in that reservoir is its residence time.

Explore More

Use this resource to answer the questions that follow.

https://www.youtube.com/watch?v=W-TPG72FSgI

  1. How much of Earth's surface is covered with water? What is most of this water in?
  2. What percent of Earth's water is salty? How much is fresh water?
  3. Of all of the fresh water, what percentage is ice, what is shallow groundwater, what is deep groundwater, what is in lakes and rivers and what is in the atmosphere as water vapor?
  4. How much of Earth's fresh water is available for human consumption?

Review

  1. Earth has a tremendous amount of water. So why is water sometimes a scarce resource?
  2. What are the reservoirs for water?
  3. What is residence time? What does it mean to say the residence time of water in the atmosphere is nine days?

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