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The Water Cycle - Water Science for Schools

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The Water Cycle

The U.S. Geological Survey (USGS) and the Food and Agriculture Organization of the United Nations (FAO) have created a water-cycle diagram for kids and elementary and middle schools.

Below is an expanded explanation of the water cycle for kids and teachers. We are currently working on a much more comprehensive version of this page.

Go back: The Water Cycle for Kids


You may think that every drop of rain that falls from the sky, or each glass of water that you drink, is brand new, but it has always been here and is a part of The Water Cycle. The water cycle describes how water is not only always changing forms (liquid water, ice, and vapor (gas)), but also moving around all over the world (above, on, and underground). This process is always happening everywhere, be it in your body or on your lawn or in the clouds or in the swimming pool. Life on, in, and above the Earth depends on the water cycle.

The heat of the sun provides energy to make the water cycle work.

The energy the sun provides is heat, which is needed to break the molecular bonds that hold water molecules together (liquid).

The sun evaporates water from the oceans into water vapor.

Since over 70% of the Earth's surface is covered by water, that is where most of the evaporation of water occurs.

This invisible vapor rises into the atmosphere, where the air is colder.

The air is actually full of water, in the form of invisible water vapor. Air nearer the Earth's surface is warmed and thus rises, taking the water vapor with it.

The water vapor condenses into clouds.

Colder air does not hold water vapor as a gas as easily as warmer air, so as the warmer air rises up into the atmosphere, it gets cooled and the water vapor in it starts to condense back out into tiny liquid cloud particles.

Volcanoes produce steam, which forms clouds.

Though not a large part of the water cycle (unless we get a major eruption), volcanoes release steam, coming from water and the components of water from deep inside the earth.

Air currents move clouds all around the Earth.

The Earth's atmosphere is never quiet - the air is moving all around all the time, often in constant patterns, such as from over oceans to over land. The moving air currrents take clouds with it, thus moving the water cycle along.

Water drops form in clouds, which then fall to Earth as precipitation (rain and snow).

As water vapor condenses into clouds, the tiny cloud droplets can combine to form larger cloud drops, which will eventually become heavy enough to come down as rain, snow, and other precipitation.

In cold climates, precipitation builds up as snow, ice, and glaciers.

 

Snow can melt, becoming runoff, which flows into rivers, the oceans, and into the ground.

 

Some ice evaporates directly into the air, skipping the melting phase (sublimation).

 

Rainfall on land flows downhill as runoff, providing water to lakes, rivers, and the oceans.

 

Some rain soaks into the ground, as infiltration, and, if deep enough, recharges groundwater.

 

Water from lakes and rivers can seep into the ground.

Water moves underground because of gravity and pressure.

Groundwater close to the land surface is taken up by plants.

The top layer of the ground is more porous (dirt, rather than hard rock) and water that falls on the land surface and seeps into the ground can saturate, at times, this layer. This layer is where plants have their roots, so they can absorb this water.

Some groundwater seeps into rivers and lakes, and can flow to the surface as springs.

Water is always moving below your feet. Rain and water on the land suface seeps (infiltration) into the ground and moves not only downward due to gravity, but also laterally. So, groundwater moves along underground along pathways such as cracks and open spaces in the rocks. Sometimes these pathways intersect the land surface, and groundwater emerges as springs and as water coming into streambeds, lakes, and eventually, the oceans.

Plants take up groundwater and evapotranspire, or evaporate, it from their leaves.

Because of some of water's unique properties, such as adhesion, capillary action, and the electrical bonds in water molecules, plants pull up water from the ground through their roots and take it all the way to the leaves, where it aids in photosynthesis, which provides food for the plants.

Some groundwater goes very deep into the ground and stays there for a long time.

Some water soaking into the ground can continue downward (due to gravity) and occupy tiny cracks and fissures in subterranian rock.

Groundwater flows into the oceans, keeping the water cycle going.


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URL: http://water.usgs.gov/edu/watercycle-kids-text.html
Page Contact Information: Howard Perlman
Page Last Modified: Monday, 17-Mar-2014 11:04:16 EDT