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6.1: Distribution of Water on Earth

Difficulty Level: Basic Created by: CK-12
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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 Figure below shows, only 3 percent of Earth’s water is fresh. 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.

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

  • reservoir: 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.

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.

Practice

Use this resource to answer the questions that follow.

http://ga.water.usgs.gov/edu/earthwherewater.html

  1. What percentage of Earth's water is usable for humans?
  2. How much of the Earth's water is ocean water?
  3. How much freshwater is in glaciers?
  4. How much freshwater is groundwater?
  5. How much freshwater is in lakes?

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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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.

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Difficulty Level:
Basic
Grades:
6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Aug 29, 2016
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