Table of Contents

- Introduction

- Precipitation

- Condensation

- Runoff and Storage

- Evaporation

- The Water Cycle

- Questions

- Glossary

- Credits

The Water Cycle

Published by K12 Handhelds, Inc.
Portal, AZ
www.k12handhelds.com

Copyright © 2007 by K12 Handhelds, Inc. License CC BY, This work is licensed under a Creative Commons Attribution 3.0 United States License.

Introduction

The water cycle is the process through which water moves above, on, and under the Earth. This is also called the hydrologic cycle (hydro means water.) Clouds, rain, snow, rivers, lakes, and evaporation are all part of the water cycle.

All the water in the world has moved through the water cycle many, many times. There is a constant amount of water on Earth. There was the same amount of water on Earth a thousand years ago as there is today.

Water on Earth is found in many places, including:

- Lakes

- Rivers

- The oceans

- Manmade containers, like reservoirs, tanks, and water systems

- Under the ground

- In the air (in clouds, rain, and snow)

- The way that water moves among these different places is the water cycle. The Sun is the main source of energy that causes the changes in water on Earth.

Precipitation

Precipitation is when water in some form falls onto the Earth. Rain, snow, sleet, and hail are all examples of precipitation.

Rain is a kind of precipitation. (2)

A cloud is a bunch of particles of water. In a cloud, water is in the form of vapor. It is floating in the air like a gas.

Clouds like this contain water in the form of vapor. (3)

When the water accumulates and the conditions are right, the water falls to earth. The water changes from a vapor to a liquid or a solid.

If it is above freezing, the result is rain. If it is below freezing the result is snow. If it is colder in the clouds than on the ground, sleet or hail may fall.

Condensation

Have you ever come outside in the morning to find the ground wet with dew? This is because water from the air has come to earth in the form of condensation. This is when water that is in a vapor form changes to a liquid form.

Dew on plants happens because of condensation of water from the air. (4)

If you hold a glass over a steamy cup of hot liquid, you will see water form on the glass. This is condensation. The vaporous water is condensing or changing to liquid on the cooler glass.

Runoff and Storage

When water falls to the Earth, it collects in pools and puddles. Some water goes into in lakes, rivers, or streams. Water forms small streams that feed into larger streams. These larger streams go into rivers. Eventually, much of this water flows to the ocean. This process is called runoff.

As water flows downhill in streams and rivers, it carries dirt with it. Over thousands of years, the water can wear away rocks and carve canyons and valleys in the land. This process is called erosion.

Much of the water on Earth is stored in the oceans. In fact, over 97% of all water on Earth is in the oceans. The water in the ocean is salty.

Most of the water on Earth is in the oceans. (5)

Some water is stored as ice at the North and South Poles. Glaciers are huge pieces of ice that stay frozen all the time.

Some water is captured by people and stored for a variety of uses. We use water to drink, to make food, to wash with, to water crops, and to give to animals. Water is essential to every living thing.

There is also a lot of water under ground. This is called groundwater. Most groundwater is stored in the cracks and spaces between the rocks and the soil. Most of the water people use comes from groundwater.

* BONUS: The water table is the top part of the ground that is saturated with water. When the water table gets too high, there may be flooding. People often drill into the water table with wells to get drinking water. If the water table gets too low there may be a drought.

Evaporation

When water is stored, some of it evaporates. This means that it turns back into a vapor and goes into the air. If you have ever left a pan of water outside and come back to find the water gone, it may have evaporated.

Heat causes evaporation to happen more quickly.

The heat of the stove makes water in the kettle boil and evaporate. (6)

The Sun is the main source of energy that causes evaporation on Earth. The hotter the Sun is the faster evaporation happens. Sometimes, in the summer, after a big rain shower, the Sun comes out and dries everything up quickly. The more heat of the Sun that gets to the Earth, the more water evaporates.

Air blowing over a wet surface also makes evaporation happen faster.

Water evaporates from the oceans every day. It forms clouds in the atmosphere and eventually falls back to the Earth as rain.

The Water Cycle

Water goes through each of these stages – precipitation, runoff, storage, and evaporation – many times. One drop of water may come down as rain, runoff into a stream, and evaporate back into the air again thousands of times. This is called the “water cycle.” It is called a cycle because it is like a circle that goes around and around over and over.

(7)

People have an effect on the water cycle by the actions they take. People build dams and reservoirs to change the flow of water. Sometimes dams are built to protect areas from flooding or to generate electrical power. Reservoirs can be built to store water for drinking and watering plants. They can also be used for recreation like boating and swimming.

Sometimes water is polluted. Factories sometimes dump trash or chemicals into the water supply. Even trash that people throw on the ground can ultimately end up in streams, rivers, or the ocean. Pollution is harmful to plants, animals, and people.

All human life needs water. We drink water. Plants and animals that provide our food need water to grow. We use water to cook and clean. We enjoy water for activities like swimming, waterskiing, fishing, and boating.

Water is a critical part of our environment.

Questions

1. Describe the water cycle.

2. What is precipitation?

3. What are three types of precipitation?

4. What is condensation?

5. What is evaporation?

6. What is the main energy source that causes change in water on Earth?

7. What makes evaporation happen faster?

8. Where is most of the water on Earth stored?

9. How do people affect the water cycle?

10. How is the water cycle important to human, plant, and animal life?

Glossary

condensation – the process by which water changes from a vapor form to a liquid form

dam – a barrier to stop or confine the natural flow of water

(1)

drought – a long period of low rainfall

erosion – the process by which water eats away at rock and soil, changing the shape of the land

evaporation – the process by which water changes from a liquid form to a vapor form

groundwater – water that is stored in the cracks between rocks and soil under the Earth

hydrologic cycle – another term for the water cycle

precipitation – water that falls to Earth as rain, sleet, hail, or snow

reservoir – a manmade lake made to store water

runoff – the amount of precipitation that reaches land and eventually reaches streams, rivers, and the ocean

vapor – a substance in the form of a gas; a substance suspended in the air

water cycle – the process by which water moves from the air to the Earth and then back to the air again

water table – the top part of the Earth that is saturated or filled with water

Credits

1 – Eric Chan, CC BY

2 – John Picken, CC BY

3 – Karen Fasimpaur, CC BY

4 – Jasmine Ramig, CC BY

5 – Peter Kaminski, CC BY

6 – Peter Harris, CC BY

7 – John M Evans, USGS

cover - US Energy Information Administration, Public Domain