The Water Cycle
If water were not continuously cycling among its three states, the world’s stores of freshwater quickly would become depleted or too polluted to use. Fortunately, our supply of freshwater continually is collected, purified and redistributed as part of the water cycle. Also known as the hydrologic cycle, this continuous process replenishes our water sources through precipitation (rain, mist, snow and sleet, for example). Some water from precipitation soaks into the ground. The rest runs off into streams, lakes and the oceans. Heat from the sun causes water to evaporate from the land and from bodies of water. Water vapor collects in the atmosphere until there is too much for the air to hold in clouds, leading once again to rain or snow. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the water cycle, transpiration also is significant. For example, a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day.
Water also cycles through living organisms. Transpiration is the loss of water from parts of plants. Water evaporates through tiny pores in leaves and stems. This process creates a pressure change that draws water and nutrients up from the roots into other parts of the plant.
While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the water cycle, transpiration also is significant. For example, a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day.
Session 2: Looking at Results
- Have students observe their boxes without removing the covers. Ask them to note the changes that have occurred inside the boxes. Ask, What happened to the ice cubes? What else is different about the inside of the box? In most cases, at least a few drops of water will have condensed on the inside of the covering. Ask, Where did the drops of water come from?
- Help students understand that all three states of water have been present in their shoeboxes. Review the different states in which water can be found—ice or snow (solid), liquid water and water vapor. Breathe on a mirror or piece of glass to show students how water vapor condenses on a surface, OR boil a small container of water, so that students may observe the cloud of steam. Hold a glass or mirror above the steam.
- Let the students remove the covers from their boxes. Ask them to observe the surface of the sand. Ask, Has the surface of the sand changed? In what ways?
- Talk about where the water in the box has gone. Ask, Where was all of the water in the box when we started? Where is the water now? If students have not noticed that the surface of the sand is wet, point out that some of the water has run into the bottom of the box to make a “lake,” and some has soaked into the sand. Help students understand that the same processes take place outside when it rains and snows.
- Have students draw a side view of the box on the remaining half of their folded sheet of drawing paper. Discuss the outcomes they observed and compare their findings with their predictions.
- Challenge students to think about what would happen if other substances (for example, chemicals, oils, etc.) also were present on or in the sand.
- Give each student a copy of “The Water Cycle” page, or project an overhead transparency of the page. Have students identify the forms in which water is present in the diagram (for example, snow on mountaintops is a solid form of water, and water evaporating from the ocean represents water in a gas phase). Direct very young students to place a sticker everywhere on the page where they can find some form of water.
Keywords: condensation | evaporation | ground water | ground water | hydrologic cycle | lake | ocean | physical science | polarity | precipitation | properties of water | river | three states of water | transpiration | water | water cycle | water molecule | water source | water supply | lesson
- Moreno, N., Tharp, B., and Dresden, J. (2011) The Science of Water Teacher’s Guide. Baylor College of Medicine: Houston.
- Illustration © Baylor College of Medicine\M.S. Young.
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Funded by the following grant(s)
My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education