To analyze the distribution of water on Earth and to recognize that the rivers and streams used to produce hydroelectricity are a small percentage of Earth's total water.

About 1.3 billion cubic kilometers of water exist at and near the Earth's surface. Most of this water - around 97 percent - is found in the oceans and seas. The remaining 3 percent is mostly frozen in huge masses of ice in the polar regions. Groundwater is the next most abundant reservoir of water. Water in lakes, rivers, and the atmosphere is less than one-tenth of a percent of the total amount of water on the Earth.

Even though rivers can be found on all continents, they make up only one millionth of the water on Earth. Despite this small percentage, the moving water of rivers is an important source of energy for producing electricity. In fact, hydroelectric energy is the most widely used form of renewable energy, providing 6% of total U.S. electricity in 2010.

Students examine a model of the distribution of Earth's water to gain an understanding of where most of the water on Earth is found and what proportion of water on Earth is in rivers and streams.

Five 4-liter or 1-gallon (clean) plastic milk jugs, five small cups or other containers, graduated cylinder, water, calculator

- Fill five 4-liter milk jugs with water. You may choose to use 1-gallon milk jugs, which work equally as well. You should note, however, that there are only about 3.8 liters (3785 milliliters) in a gallon milk jug. For the purposes of this demonstration, they can be considered to hold 4 liters.
- Tell students that these five jugs of water represent all the water on Earth.
- Have students calculate how many milliliters are in the 20 liters (five milk jugs).

*There are 1000 milliliters in 1 liter. Therefore, there are 20,000 milliliters in 20 liters.* - Have students examine the following table:

Distribution of Earth's Water Reservoir Percentage of Total Water Oceans 96.54 Ice (mostly galciers) 1.74 Groundwater 1.69 Lakes 0.013 Atmosphere 0.001 Rivers 0.0002

** These figures account for 99.9 percent of all water. They do not add up to 100 percent because some water is tied up in the biosphere and as soil moisture.*

Source: USGS - Tell students that ice (mostly in the form of glaciers) holds 1.74 percent of all the water on Earth. Have students calculate the number of milliliters that represents the water found in glaciers.

*20,000 milliliters x 0.0174 = 348 milliliters* - Use a graduated cylinder to remove this amount from the milk jugs and pour it into a separate container labeled "glaciers."
- Repeat steps 5 and 6 for the water found in groundwater, saltwater and freshwater lakes and streams, and the atmosphere.

*Ground water: 20,000 mL x 0.0169 = 338 mL*

Lakes: 20,000 mL x 0.00013 = 2.6 mL

Atmosphere: 20,000 mL x 0.00001 = 0.2 mL

Rivers: 20,000 mL x 0.000002 = 0.04 mL -
Have students calculate the number of milliliters of water in the oceans, but leave the water in the milk jugs.

*Oceans: 20,000 mL x 0.9654 = 19,308 mL* - Have students develop their own models that show the percentages of each category of water using the values in the table. They should use something other than water as the physical material for their models. For example, they could cut up a sheet of graph paper to represent the proportion of water in different reservoirs.
- Have students consider the following:
- What observations or discoveries did you find most surprising or striking?

*Most students have observed or experienced a river or stream. They are often surprised by the small volume of water in rivers.* - How would you describe the use of rivers for hydroelectricity as a percentage of Earth's total water?

*The volume of water in rivers is a small percentage of Earth's total water. Not all rivers are used for hydroelectricity. Therefore, rivers used for hydroelectricity are a minute fraction of Earth's total volume of water.*

- What observations or discoveries did you find most surprising or striking?