Make a Miniature Water Cycle Model

Introduction

Have you ever wondered where the rain that falls from the sky comes from? You might say the rain falls from the clouds, but what are clouds and where do they come from? All these questions and many more can be answered by looking into how water moves on land and in the atmosphere, which is described in the water cycle. In this activity, you will investigate some of the processes that make water move in and out of the atmosphere by making a miniature water cycle model inside a plastic bag.

Materials

• Re-sealable snack-size plastic bags (2)
• Permanent marker
• Water
• Liquid measuring cup
• Tablespoon
• Strong tape, like duct tape
• Blue food coloring
• Area that gets lots of sun and another that gets very little sun during the day
• Optional: Sand or dirt
• Optional: Larger pieces of rock that will fit in the bag, at least 2

What Happened?

For this activity, you basically built a little water cycle model inside a plastic bag. The water that you added to the bag represented the water bodies we have on our planet, such as the ocean, lakes, or rivers. The sand represented the soil or land around these water bodies, and the rock represented a mountain. In the bag that you taped to the sunny window, you should have observed that over time, tiny water droplets formed on the sides inside the plastic bag. If you waited long enough, these droplets should have become bigger and bigger until they ran down the sides to the bottom of the plastic bag. What you saw happening inside the bag was a combination of evaporation, condensation, and precipitation, which are all processes that happen in the water cycle.

As the sun heats up the water inside the bag, the water evaporates, which means it turns into water vapor. You probably were not able to see the water vapor because it is a colorless gas and difficult to see. However, as the water vapor rose into the air above the water and cooled down again, it condensed back into liquid water. The water droplets that you observed are the result of this condensation. As more and more water evaporated and condensed onto the plastic bag, the water droplets inside the bag should have become bigger and bigger until they got heavy enough to flow down the sides of the bag due to gravity. This process is called precipitation and we can observe it every day that raindrops fall from the sky.

If you added the sand and the rock to the bag, you should have also been able to observe runoff and infiltration, which are two more processes within the water cycle. You probably noticed that the water droplets that fell onto the sand got soaked up by the sand, similar to rain falling onto soil and getting soaked up by it. This process is called infiltration. Rain drops that fell onto the rock were not soaked up but flowed down the rock until they reached the ground or the water inside the bag. Water that does not soak into the ground and instead flows over the land is called surface runoff. It keeps flowing until it runs into a water body or eventually gets absorbed by the soil.

While you should have been able to observe all of these processes in the bag on the sunny window, you probably noticed that these processes did not occur in the bag in the shade. For the water cycle to happen, there needs to be a heat source, such as the Sun, that causes the water to evaporate. Without the heat from the Sun, the water cycle would come to a stop.

Digging Deeper

Earth is a planet full of water. About 70% of Earth's surface is covered by water, but water is not distributed equally on Earth. 97% of the water is found in the world's oceans; the remaining water is found in glaciers and ice, rivers, lakes, underground, or in the atmosphere. The U.S. Geological Survey (USGS) estimates that Earth's total water supply is about 326 million cubic miles (about 1,359 million cubic km) of water! Most of this water is liquid, but on Earth water can also exist as a solid in the form of ice or snow, and as a gas in the form of water vapor.

The water on Earth is in constant movement. This movement is not limited to Earth's surface, but also includes the atmosphere and the subsurface. The water cycle, also called the hydrologic cycle, describes how water moves within and on Earth and the atmosphere, and its transition from one state to another. The water cycle is dynamic and involves many different processes that contribute to water being moved from one place to another. Some of these processes are evaporation, condensation, precipitation, transpiration, and infiltration (Figure 1).

The Sun is the major driver of the water cycle. Surface waters such as the ocean, lakes, and rivers are heated up by solar energy and thus, some of the liquid water evaporates and becomes water vapor. As warm air rises, that moist air rises to higher altitudes. In the process, it cools down and eventually condenses into water droplets. This process is called condensation. We see these tiny water droplets as clouds in the sky. The tiny water droplets within a cloud merge and become bigger and heavier until they get too heavy and fall down to the ground due to gravity. This process is called precipitation. If temperatures are too cold to keep the water droplets in a liquid state, they crystalize and form snow or ice crystals and fall to the ground as snow or hail. Snow that is deposited on mountains or glaciers can be stored as solid water for thousands of years until it gets warm enough for it to melt again. Ice and snow can also directly transform into water vapor in a process called sublimation. Gravity also causes liquid water to fall on land, and thus get absorbed into the soil through infiltration where it can be stored as groundwater or taken up by plants. Plants release some of the water back into the atmosphere through their leaves in a process called transpiration. Water that does not soak into the ground flows over the land as surface runoff until it enters a river, lake, or the ocean. Again, gravity is the driving force. These water bodies function as storage reservoirs for liquid water. It is from there that the water cycle starts again when the Sun's energy turns the water into water vapor.

The horizontal movement of water in the water cycle happens both in the atmosphere through clouds being moved around the globe by air currents, and through water flow on land as surface runoff or as waterflow in rivers or underground. Although water in the water cycle is constantly moving and continuously transforming from one state to another, no water ever disappears! The water that exists on our planet today is the same water that existed on our planet a thousand or a million years ago! The natural cycling of water is one of the most important processes on Earth, as it provides all living organisms with a continuous supply of fresh water. The water cycle also plays a significant role in the weather patterns on our planet, as it contributes to weather events such as precipitation or cloud formation. Without the water naturally recycling itself, life on Earth would not be possible!

For Further Exploration

• Research other process that take place in the water cycle, such as transpiration, freezing, or sublimation. Can you find a way to add these to your water cycle model?
• What do you think would happen if you changed the temperature of the water that you add to your bags? Try it and see. Does it make a difference if the water is hot or cold?
• Try to build a larger and more-realistic water cycle model. Instead of using a small plastic bag, can you build a water cycle model in a larger plastic box? How could you model the Sun, the water bodies, the land, or mountains in this model?

Related Resources

Project Ideas

Activities

Lesson Plans

Links

• Water Facts - Worldwide Water Supply, from Bureau of Reclamation
• The Water Cycle for Adults and Advanced Students, from U.S. Geological Survey (USGS)
• A Multi-Phased Journey, from NASA Earth Observatory

Careers