Water Cycle

Target Curriculum: Science

Target Grade: 3 - 6

SOLs: SCI.3.9   SCI.4.1   SCI.4.6   SCI.5.1   SCI.5.4   SCI.6.1  
Time: 20 minutes to assemble, plus observation time

Objective:
This activity helps students understand how water moves through the Earth's many ecosystems. It may be done as a class demonstration or in small groups.

Purpose:
Students will create a model of the water cycle in a closed environment to gain a better understanding of the processes involved.

Materials:
Per Group

• clear plastic container with a tightly fitting lid (The large plastic containers used in salad bars work well.)
• small plastic cup
• large rock
• ice
• water
• lamp with reflector
• marker
• small cutting knife or Exactor knife

Procedure:

1. In a corner of the top of the plastic container, trace around the circumference of the large end of the plastic cup. Then, flip the cup over, center it inside the first circle, and trace around the small end of the cup. Make a new circle halfway between the first two circles. Flip the container over so that the lid rests on a good cutting surface. Cut out the middle circle. The cup should rest about half-way down in the hole.
2. Pour enough water into the plastic container to cover the bottom to a depth of 2-3 cm.
3. Place the rock directly under the cup hole. Make sure the rock is large enough to rise above the water level.
4. Fill the plastic cup with ice and slide it into the hole in the container's lid.
5. Position the gooseneck lamp so that it shines down on the water at the far end of the container.
6. Measure the initial water level. Observe the model over a period of time, measuring the water level from time to time.

Observations:
What happens in the container?

In this activity, what does the lamp represent? The cup of ice? The rock?

What causes the water droplets to form on the cup?

Was any water lost in this activity? How do you know?

Conclusions:
Why does evaporation occur in the Earth's water cycle?

Why does condensation occur?

For Your Information:
The sun's energy heating the Earth causes water to evaporate. The resulting water vapor rises and meets a cooler air mass. The water vapor condenses and forms water droplets, which fall as precipitation. This unceasing process is called the "water cycle," or the "hydraulic cycle."

In this activity, the environment in the plastic container simulates the water on the Earth. The lamp, which heats the water in the box, represents the sun. The water from the "ocean" evaporates and then condenses when it hits the cup of ice, which represents the cooler air masses in the upper atmosphere. After some time, the condensed water vapor falls to the "Earth" again and makes its way back to the "ocean."

Once the water cycle has begun to flow, the water level should remain about the same in the box. The water in the Earth's environment is constantly being cycled and recycled.

Extension:
Measure the temperature of the water in different areas within the container. (For example: under the lamp, behind the rock, in between the rock and the lamp, etc.)

Class Discussion Questions:

1. How do these temperature differences help to account for the different rates of evaporation on the Earth?

   Set up a second water-cycle model in which a small section of the lid has been cut away. Be sure to measure the water level in both models before any evaporation occurs. Leave the models in place for about a week. Measure the water levels daily. How do the results compare?

2. Is the Earth's water cycle system closed? This water cycle activity was first developed for a weather workshop in November of 1997. At that time, scientists described the Earth's water cycle system as closed with a fair degree of confidence. However, this spring, a scientist named Louis A. Frank at the University of Iowa presented evidence that water-in the form of 20-40 ton snowballs-may be entering the Earth's atmosphere every day and adding to the Earth's water supply. If this proves to be true, scientists will have to reconsider many of their ideas about the planet's water cycle. (You may read more about this discovery in the Washington Post article entitled "Cosmic Snowballs Detected Pelting Earth's Atmosphere" that was published on Thursday, May 29, 1997.)