The Water Cycle

Did you know that you could be drinking
the same water that a dinosaur once drank?!

The very same water that is on the Earth now, 
was on the Earth hundreds of millions of years ago.

Water never stops moving. Snow and rain fall to the earth from clouds. 
The rain and melted snow run downhill into rivers and lakes, 
sometimes crashing over waterfalls. 
Eventually the water flows into the ocean.

During evaporation, the water turns from liquid into gas 
and moves from oceans and lakes into the atmosphere 
where it forms clouds. 
Then the cycle begins all over again.

Look at the word cycle in the title above.  
Like a circle or the wheels on a bicycle, 
the term water cycle means that 
the water on our Earth keeps going around and around. 
It never stops moving. 
The water cycle is 
the Earth's fabulous, automatic, giant recycling project. 

All plants and animals need water. 
The Earth uses its supply of water and then recycles it. 

And it is all controlled by the sun. 
The sun's energy gives us heat.

The heat warms the water that it touches, 
and eventually that water evaporates.

The water that has evaporated is no longer a liquid. 
It is a gas, called water vapor.

When plants give off water vapor into the air, it is called transpiration.

Did you know that air high up on a mountain is cooler than the air 
near the surface of the Earth? 
Well, the water vapor rises. 
As it gets higher, the water vapor collects 
and starts to cool because the air around it is cooler. 
That is how clouds are formed.  

When the water vapor cools down enough, 
it changes back into liquid water. 
That is called condensation. 
When the drops of water become large enough, 
the air can no longer hold them. 
They start to precipitate (fall) out of the sky. 
Precipitation can be rain, or snow, or sleet, or hail.

What happens next?
The water that hits the ground may seep through the soil.  
That is called infiltration. 

Or... if the water falls in the mountains or near the coastline, 
it may run off into lakes, rivers, or oceans. 
As the water runs over the land, 
it collects minerals and other nutrients 
that are in the soil. 
That is why river valleys have soil that is so good for growing crops. 

 TEST YOURSELF NOW   
Drag the correct water cycle definition next to its matching term.

...And that is how the whole process starts again 
as the water starts to evaporate from the oceans.
 
  Copy and paste this diagram into Paint.  
  
Label:   rain - lake - river - ocean - runoff - evaporation - 
transpiration - infiltration - condensation

OR...

Do the ABC/Teach Water Cycle Project at home.

Go to Brainpop to learn more about the water cycle.

Play Droplet and the Water Cycle game. Click here.

Go here to read The Story with No End--or, the biography of a drop of water.
Make your own water cycle. Click here.

Precipitation, evaporation, and transpiration are all terms that sound familiar, yet may not mean much to you. They are all part of the water cycle, a complicated process that not only gives us water to drink, fish to eat, but also weather patterns that help grow our crops.

 Water is so very important to life on this planet. It is an odorless, tasteless, substance that covers more than three-fourths of the Earth's surface. Most of the water on Earth, 97% to be exact, is salt water found in the oceans. We can not drink salt water or use it for crops because of the salt content. We can remove salt from ocean water, but the process is very expensive.

Only about 3% of Earth's water is fresh. Two percent of the earths water is in solid form, found in ice caps and glaciers. Because it is frozen and so far away, the fresh water in ice caps is not available for use by people or plants. That leaves about 1% of all the Earth's water in a form useable to humans and land animals. This fresh water is found in lakes, rivers, streams, ponds, and in the ground. (A small amount of water is found as vapor in the atmosphere.)

Evaporation

Evaporation is the process where a liquid, in this case water, changes from its liquid state to a gaseous state. Liquid water becomes water vapor. Although lower air pressure helps promote evaporation, temperature is the primary factor. For example, all of the water in a pot left on a table will eventually evaporate. It may take several weeks. But, if that same pot of water is put on a stove and brought to a boiling temperature, the water will evaporate more quickly.

 During the water cycle some of the water in the oceans and freshwater bodies, such as lakes and rivers, is warmed by the sun and evaporates. During the process of evaporation, impurities in the water are left behind. As a result, the water that goes into the atmosphere is cleaner than it was on Earth.

Precipitation

When the temperature and atmospheric pressure are right, the small droplets of water in clouds form larger droplets and precipitation occurs. The raindrops fall to Earth.

As a result of evaporation, condensation and precipitation, water travels from the surface of the Earth goes into the atmosphere, and returns to Earth again.

Condensation is the opposite of evaporation. Condensation occurs when a gas is changed into a liquid. Condensation occurs when the temperature of the vapor decreases.

When the water droplets formed from condensation are very small, they remain suspended in the atmosphere. These millions of droplets of suspended water form clouds in the sky or fog at ground level. Water condenses into droplets only when there are small dust particles present around which the droplet can form.

Much of the water that returns to Earth as precipitation runs off the surface of the land, and flows down hill into streams, rivers, ponds and lakes. Small streams flow into larger streams, then into rivers, and eventually the water flows into the ocean.

Surface runoff is an important part of the water cycle because, through surface runoff, much of the water returns again to the oceans, where a great deal of evaporation occurs.

Infiltration is an important process where rain water soaks into the ground, through the soil and underlying rock layers. Some of this water ultimately returns to the surface at springs or in low spots downhill. Some of the water remains underground and is called groundwater.

As the water infiltrates through the soil and rock layers, many of the impurities in the water are filtered out. This filtering process helps clean the water.

As plants absorb water from the soil, the water moves from the roots through the stems to the leaves. Once the water reaches the leaves, some of it evaporates from the leaves, adding to the amount of water vapor in the air. This process of evaporation through plant leaves is called transpiration. In large forests, an enormous amount of water will transpire through leaves.

Let's Review!

1.  First the water from the Earth's surface evaporates.  Then it rises into the atmosphere, is cooled, condenses, and forms clouds.

2.  When enough water collects in the clouds, they release moisture  in the form of rain, sleet, snow, or hail.  And once again, the water returns to the Earth.

3.   The water that's fallen to the Earth runs off into lakes, rivers, streams, and any other body of water.  This water will eventually seep through layers of the Earth's surface  where impurities filter out.

4.   Then, the water is heated by the sun and evaporates,  and the whole cycle begins again.