Margaret D. Andrews
Scientists aren’t sure exactly when liquid water first appeared on our planet, but they estimate it was over one billion years ago. After the planet was formed it was too hot for most substances to exist in other than a gaseous form, but as the temperature decreased many of the chemicals that are now a part of the earth were formed from these gases. Water formed from the gases of hydrogen and oxygen. This newly formed water fell as showers of hot water until enough rain had fallen that water covered most of the planet and made our first oceans. These first rains also carried many other compounds out of the atmosphere into the oceans just as today’s rain washes pollution out of the sky. The first rain started the water cycle and it has been running ever since.
The hydrological (water) cycle can be divided into four parts or phases. All the water in the world is somewhere within this cycle, although some of it can be shunted temporarily out of the main pathways for a time. The four phases of the major cycle include: precipitation, earth water, evaporation, and air water. Water that has been taken up and made a part of a living organism (be it a plant or an animal) is temporarily pulled out of this cycle, but after helping the organism grow, transport food or eliminate wastes, water reenters the cycle. Water that moves into the ground water system, into the deep ocean, or is frozen into a glacier can take thousands of years to move into the next phase of the water cycle because it is effectively isolated from the rest of the cycle but eventually it will come back into the process.
Water is never really standing still, even when we see it in a lake or in the ocean. Rain falls out of the sky and is either absorbed into the ground to become part of the ground water or it runs over the surface of the ground until it enters a small stream or brook and becomes part of the surface water system. Streams run together and form rivers which empty into lakes or oceans. From these places, water evaporates and moves back to the clouds. The fog visible over lakes or the ocean early in the morning is the evaporation process made visible. Rain water trapped in puddles also evaporates right back to the clouds. Evaporation is the reason the oceans don’t overflow. Water also returns to the clouds through the processes of living organisms. When animals breathe, or when they perspire, they give off water vapor. Plants have a similar mechanism called transpiration. Water taken up by the roots is pulled up through the plant in tiny tubes, known as xylem, all the way to the leaves where it leaves the plant by evaporating through tiny pores. The evaporation of water through the leaves is what provides the force to move the water up the plant and, while it doesn’t seem like a big deal, it involves a lot of energy. It’s been estimated that the energy required to move water from the roots to the leaves of a redwood would be great enough to boost a can of soda into low planetary orbit!
There are some things that determine in which section of the liquid phase precipitation ends up. Topography is an important factor. Tall mountain ranges create a “rain shadow”. When the moisture laden air hits the mountain, the air is forced up to higher elevations where it cools, causing all the condensation and precipitation to occur on that side of the mountain. Thus in Washington state, there is a temperate rain forest on the western side of the Cascades and a desert on the eastern side.
Learning Activities
The Adventures of Drip Drop—a story to read out loud.
Once upon a gloomy day, far above a town similar to yours, and excited raindrop sat in his cloud waiting to fall to Earth. He continued to gather water until he was so heavy he began to fall. This was Drip Drop’s favorite part of the water cycle. He knew when he landed, it could be in one of several exciting places around the world. At the thought of this, Drip began dancing around the world. At the thought of this, Drip began dancing and twirling in a playful manner. His thoughts wandered on his destination:
He’d once landed in the savannas of Brazil
Where his fine mist helped water the plants.
One time he landed on a great plateau
Where Indians rejoiced with a dance.
Maybe he’d land in China this time,
Where he’d water the fields of rice.
He’s always wanted to fall near Alaska
And become part of the glacier ice.
He could splash into a German country pond
To make a home for frogs and fish.
He may even land in an English King’s fountain
Where Children throw coins for a wish.
How much fun it would be to land in the alps
Where he’d turn to mountain snow.
And if he fell in the Nile in Egypt,
Down the world’s longest river he’d flow.
Drip’s thoughts were broken by the warmth of the sun above and a strange feeling came over him. As the sun’s rays passed through Drip’s body, beautiful colors began to appear: violet, indigo, blue, green, yellow, orange, and red. Drip noticed the same thing happening to all of the other raindrops around him. Together, they had formed a spectacular rainbow that stretched across the sky.
There was a great celebration among the drops with all of them splashing and singing. Then suddenly, the colors began to fade, and Drip was close enough to see the ground below him.
He heard the voices of children laughing and playing. Then PLOP! Drip Drop was surrounded by old raindrop friends. He could feel salt mixing through his body and the tide pulling him toward a sandy beach. Then, CRASH! He was being pulled out toward the sea again. Drip Drop had landed in the Mediterranean Sea!
He was ecstatic over his destination and spent many sunny days in the sea until he evaporated and became part of the water cycle again.
The Changing Forms of Water
a. Wet Rocks and Blackboards
Materials:
*sponges
*hot and cold water
*several small flat rocks, different colors if available
*stopwatch
Using sponges, make streaks with hot and cold water on the blackboard.
Ask which one will disappear faster
.
Time and record the results. Make more streaks with water at the same temperature and have someone fan on streak to see what happens.
Ask why one streak disappeared faster than the other.
If the blackboard is portable, arrange it so one side is in direct sunlight and the other side is shaded and make two more streaks.
Ask where all the water is going
.
You can perform similar experiments with different colors of flat rocks (dipped in hot or cold water, placed in sun or shade, waived or kept still).
b. Tea Kettle Rainstorm
Materials:
*hot plate or water heating instrument
*tea kettle
*2 tea cups or a cup and bowl
*hot water
*oven mitt or winter glove
Half fill the kettle with water and heat it over a hot plate or stove. When water starts to boil, steam will come out the spout in a stream. Hold one tea cup upside down where the steam stream will hit it.
Wear a glove or be very careful of your fingers!
Place the other cup or a bowl under the upside down cup. As the steam hits the first cup it should condense and drip into the second cup.
Ask which part of the demonstration represent the rain, a lake, the sun and other parts of the water cycle
.
C. Hot Water Bottles and Ice Cubes
Materials:
*narrow neck bottle or thermos
*hot water
*ice cubes
*dark background
By holding an ice cube over the mouth of a thermos or a narrow necked bottle full of hot water, you can create a fog cloud. The cloud will show up best in indirect light against a dark background.
d. Water Cycle Microcosm
Materials:
*clear glass or plastic container
*plastic wrap
*water
*small weight
*cup that fits completely inside container
Place water in the bottom of the clear container and set the cup in the center of the container. The cup should not float, but rest steady on the bottom. Cover the container loosely with a piece of plastic wrap but make sure the edges are sealed. Place the small weight in the middle of the plastic wrap so that it lines up with the cup in the container. Place the whole thing in direct sunlight where it can absorb heat. After a short time, water should start condensing on the underside of the plastic wrap, roiling to the point under the weight and dripping off into the cup, forming a water cycle. If you have enough materials, you can have several small groups make different microcosms and place them in different areas to see how fast the cycle can be started. Note: If you start this with hot water, it works much faster.
