Earth is the only planet with liquid water at its surface.

1. Earth’s Water Oceans97.2% Earth is the only planet with liquid water at its surface. Glaciers2.1% (including water vapor, groundwater, lakes, and rivers) Other0.7%

2. If you could look at Earth from space, you would see that most of its surface—about 71 percent—is covered with water. The temperature range on Earth allows water to occur in three states: liquid, solid, and gas. Most of Earth’s water is the salt water found in the oceans.

3. Only about three percent of Earth’s water is fresh. • Most of this fresh water occurs as ice and snow in Earth’s high mountains and polar regions. • Most of Earth’s liquid fresh water is in the form of groundwater, the water found underground. • Smaller amounts of fresh water are found in lakes and streams and as water vapor or clouds in the atmosphere.

4. What processes are involved in the water cycle? The water cycle is made up of several processes, including evaporation, transpiration, condensation, precipitation, and the eventual return of flowing water to the ocean. The Water Cycle

5. The Water Cycle The continuous movement of water among the oceans, atmosphere, and land is called the water cycle. The energy of sunlight and the force of gravity power the water cycle.

6. The Water Cycle Water is constantly moving through the water cycle. Precipitation Condensation Transpiration Evaporation Groundwater Runoff

7. The Water Cycle Evaporation Solar energy causes water on Earth’s surface to evaporate. Warm, moist air rises, carrying the water vapor higher into the atmosphere. • Evaporation occurs over the ocean and large lakes, streams, ponds, and other sources. • Water vapor also enters the atmosphere from plants in a process called transpiration.

8. The Water Cycle Condensation As warm air rises, it expands and cools. Colder air can hold less water vapor than warm air, so condensation occurs. Billions of water droplets or ice crystals make up clouds.

9. The Water Cycle Precipitation When the water droplets or ice crystals in clouds get too heavy, gravity causes them to fall to the ground as precipitation—rain, snow, sleet, hail, or freezing rain. Worldwide, the rates of evaporation and precipitation are balanced.

10. The Water Cycle Completing the Cycle A single molecule of water moves between the surface and atmosphere many times. • About 90 percent of the water that evaporates from the oceans falls back into the oceans. • Most precipitation that falls on land quickly returns to the atmosphere through evaporation and transpiration.

11. The Water Cycle • Water seeps into the soil as groundwater or becomes frozen in glaciers. A glacier is a large mass of moving ice and snow on land. • Some water that falls on land flows into streams before it returns to the ocean.

12. Where is Earth’s fresh water found? A small portion of Earth’s fresh water is located in the atmosphere, streams, and lakes. Most is located in groundwater and glaciers. Fresh Water

13. Fresh Water Only a small portion of the water on Earth exists as fresh water. Fresh water is relatively free of minerals and salts.

14. Fresh Water The Atmosphere A tiny percentage of Earth’s fresh water is found in the atmosphere as water vapor or clouds. Water vapor stays in the atmosphere for just a short time—a few hours or a few days.

15. Fresh Water Streams and Rivers Streams and rivers contain even less fresh water than the atmosphere. A great deal of water passes through streams, making them very important in shaping the surface of the land.

16. Fresh Water Runoff is water that flows over Earth’s surface. • Runoff and some groundwater flow into streams. • Streams flow together to form a network, called a river system. • A major river has many smaller streams, called tributaries, that flow into it. The area of land that contributes water to a river system is called a watershed.

17. Fresh Water The Mississippi River watershed covers most of the central United States.

18. Fresh Water Ponds and Lakes Ponds and lakes form in depressions in the land. • Lakes form in large, deep depressions. • Ponds form in smaller depressions and tend to be smaller and shallower than lakes.

19. Fresh Water Ponds form when water collects in small, shallow depressions. Many ponds support lush plant growth and animal life.

20. Fresh Water Water Beneath the Surface About a quarter of all fresh water is groundwater. • Close to the surface, the pore spaces in rock and soil are filled with air. • Deeper below the surface is the saturated zone, a region where the pore spaces are entirely filled with groundwater.

21. Fresh Water • The top of the saturated zone is called the water table. • Water flows slowly from high elevations of the water table to low elevations. • Lakes and streams are usually found where the ground is below the water table.

22. Aquifer Spring Well Impermeable layer Saturated zone Water table Saturated zone Fresh Water Groundwater makes up about one quarter of all fresh water. In the saturated zone, groundwater flows in the pore spaces in underground rock.

23. Fresh Water A rock is permeable if water can easily pass through it. Rocks like sandstone and limestone are permeable. A permeable rock layer that is saturated with water is called an aquifer. Aquifers are refilled, or recharged, as rainwater seeps into them.

24. Fresh Water Rocks are impermeable if water cannot easily pass through them. • Shale and unbroken granite are examples of impermeable rock. • Groundwater in loose rock or soil above an impermeable layer may flow out of a hillside as a spring.

25. Fresh Water Glaciers and Icebergs Glaciers form in areas where more snow falls than melts each year. • As snows build up, the weight of the snow presses on the layers below, changing them to ice. • When the weight of the snow is great enough, glaciers begin to flow slowly downhill.

26. Fresh Water • Ice is added at the top of the glacier through snowfall. • Ice is removed from the glacier by melting, sublimation, and the formation of icebergs. • Icebergs are large pieces of ice that break off when a glacier reaches the ocean.

27. Fresh Water Icebergs are formed when a piece of a glacier breaks off into the ocean. Icebergs are made of fresh water.

28. Assessment Questions • During which part of the water cycle do water molecules gain energy? • runoff • condensation • evaporation • precipitation

29. Assessment Questions • During which part of the water cycle do water molecules gain energy? • runoff • condensation • evaporation • precipitationANS: C

30. Assessment Questions • What is an aquifer? • the area of land that contributes water to a river system • the underground line separating saturated regions from unsaturated regions • a rock that water cannot easily pass through • a rock layer saturated with water

31. Assessment Questions • What is an aquifer? • the area of land that contributes water to a river system • the underground line separating saturated regions from unsaturated regions • a rock that water cannot easily pass through • a rock layer saturated with waterANS: D