Salt water 97.5%

Groundwater 20%

Ice 79%

Fresh waterAll water

Fresh water 2.5%

Salt water 97.5%

Groundwater 20%

Ice 79%

Surface water 1%

Data from the United Nations Environment Programme (UNEP) and World Resources Institute.

0133724751a012ck8/12/09

420 Lesson 1

“Water, Water, everyWhere, nor any drop to drink.” The well-known line from the poem The Rime of the Ancient Mariner describes the situation on our planet quite well. Water is everywhere—it falls from the sky as rain and snow, it covers our poles as ice, and, of course, it fills our oceans. Water even makes up about two thirds of our bodies! However, in many places on Earth, there is not nearly enough of it to meet human needs. One in eight people lack access to clean fresh water. How can there be so little of something so abundant?

Where Is Our Water? As a natural resource, fresh water is renewable. However,

quantities of fresh water on Earth are limited.

Water may seem abundant to us, but globally, water that we can actually use is quite rare. However, because fresh water is a renewable resource, its supplies can be maintained—as long as we learn to use them sustainably.

A Renewable Resource Water is considered a renew-able resource because the water cycle, or hydrologic cycle, constantly recycles it. Cycling water redistributes heat, erodes mountain ranges, builds river deltas, maintains organisms and ecosystems, and shapes civilizations. As water moves, it may change state: from solid ice, to liquid water, to gaseous water vapor, but overall, very little water is ever gained or lost. Water, however, is not always useful or where we want it.

A Limited Resource You can’t drink salt water or effec-tively water crops with ice. Most of people’s day-to-day activi-ties rely on fresh, liquid water. However, only a very small portion of Earth’s water—about one half of one percent—is both fresh and liquid (Figure 1). If a full 2-liter bottle repre-sented all the water on Earth, only about two capfuls would be fresh, liquid water.

Earth: The Water PlanetLE

SSO

N 1

FIGUre 1 Water on earth Only 2.5% of Earth’s water is fresh water. Of that 2.5%, most is frozen—tied up in glaciers and ice caps.

• Discuss how fresh water can be both renewable and limited.

• Explain the significance of a watershed.• Explain how most groundwater is accessed.

Reading Strategy Before you read, create a main idea and details chart using the sentences under blue headings in this lesson as your main ideas. As you read, fill in the chart with details from the text.

vocabulary fresh water, surface water, runoff, river system, watershed, groundwater, permeable, impermeable, aquifer, water table, recharge zone, well

Guiding Question: Where is all of our water?

14.1 LESSON PLAN PREVIEWInquiry Use a teacher demo to model Earth’s water supply.Real World Students use the United States Watersheds map to identify the watershed they live in.Differentiated Instruction Support students struggling as they identify aquifer features.

14.1 RESOURCESPaper and Pencil Activity, Watershed Boundaries • Graph It, Global Freshwa-ter Resources • Map It Online • Lesson 14.1 Worksheets and Assessment • Chapter 14 Overview Presentation

Find OutMore

Find OutMore

KEYAvailable fresh water

More than 100,00020,000–100,00010,000–20,0005,000–10,0002,000–5,0001,000–2,000Less than 1,000Insufficient dataMajor inland waterwayNational border

(cubic meters per capita per year)

Data from U.N. Environment Programme and World Resources Institute, as presented byHarrision, P., and F. Pearce. 2000. AAAS Atlas of Population and the Environment. Berkeley, CA: University of California Press.

Water Resources 421

About 97.5% of Earth’s water is salt water. Most salt water is found in the oceans and is too salty for drinking or watering crops. Only 2.5% of Earth’s water is considered fresh water, water that is relatively pure with few dissolved salts. Of that tiny proportion of water, more than three quarters is frozen in the form of glaciers and ice caps. The remaining 21%, found in lakes, rivers, the atmosphere, organisms, and soil, is liquid. Only some of this 21% is drinkable or usable for crops. Clearly, useful fresh water is a very limited resource.

People and Water Because water is so critical to our daily lives, you might expect that people would choose to live only in places with ample access to it. Unfortunately, this is not the case. Many areas with high population density, such as Las Vegas and its suburbs, do not have a lot of water. Other areas have plenty of water, but few people. These situa-tions cause inequalities in per capita water resources, as shown in Figure 2. For example, India has almost four times as much water as Australia. However, India has almost 55 times the population. So, per capita, Australia has far more water available to its residents than India does.

Seasonal Availability Fresh water is distributed unevenly in time as well as space. Although some areas have access to fresh water, it might not always be there when it is needed. Many parts of the world experience distinct rainy and dry seasons. Portions of western and central India, for example, get around 90 percent of their annual rainfall during the three-month monsoon season. Because fresh water is distributed unevenly in space and time people are often challenged with moving fresh water from its source to where it’s needed when it’s needed.

FIGURE 2 Water Availability Nations vary in the amount of fresh water per capita available to their citizens. For example the nations in purple (Iceland, Papua New Guinea, Gabon, and Guyana) each has over 100 times more water per person than do many Middle Eastern and North African nations.

Use the Internet or other resources to find out the following facts about your local water:1. What major watershed is your

hometown part of?2. Does your area experience

any seasonal changes in water availability?

3. Has your hometown experienced any water shortages in the last 10 years?

Global Freshwater Resources

ANSWERS

Find Out More Students’ responses should show they used reliable re-sources in their research.

Map it

422 Lesson 1

Surface Water Surface water is contained within watersheds.

Water is categorized as either surface water or groundwater depending on where it is located. Just one percent of easily accessible fresh water is surface water, water found on Earth’s surface. Sources of surface water include rainfall and melting snow, glaciers, and ice caps. Water travels from these sources to bodies of surface water as runoff. Runoff is water that flows over land and has not been absorbed into the ground. Runoff can flow into standing, or still, bodies of surface water such as lakes and ponds, or it can join up with a river system.

River Systems Water moves downhill according to gravity, form-ing a network of connected streams and rivers called a river system. As runoff flows downhill, it can form shallow grooves in the earth. These grooves can deepen and flow together forming streams. Streams can in turn merge into rivers. Smaller rivers, called tributaries, flow into larger rivers. Eventually, running water from a river system empties into a body of water such as a lake or the ocean.

Watersheds Pretend you are standing at a river’s mouth, where its waters meet the ocean. Now, picture tracing a drop of water back up the river, through smaller and smaller branches of the river system, right back to the original spot where it fell as precipitation. If you could do that for every drop of water that flows out of a river, you will have defined its watershed. A watershed includes all of the land area that supplies water to a particular river system. Watersheds are sometimes called drainage basins because they drain into a river system the same way that rainwater drains into street or house gutters. Figure 3 shows the major watershed regions in the United States.

ReadingCheckpoint

What is a river system?

The Mississippi River Watershed The Mississippi River Basin is the third largest watershed in the world, covering over 3 million square kilometers (1.2 million square miles). In fact, it drains 41% of the land area of the contiguous United States. Use the map in Figure 3 to answer the following questions. 1. Interpret Maps Trace the path of a raindrop that falls

in Billings, Montana, as it makes its way to the Gulf of Mexico. List the rivers it flows through.

2. Explain Why does the map of the Mississippi River watershed contain both the river system and the land that surrounds it?

3. Infer The Mississippi River watershed is bor-dered by the Appalachian Mountains to the east and the Rocky Mountains to the west. Why does it make sense that mountain ranges form natural watershed boundaries?

ANSWERS

Reading Checkpoint A network of connected streams and rivers

Map It

1. Yellowstone River, Missouri River, Mississippi River

2. Watersheds are made up of the land that drains into a river system as well as the river system itself.

3. Mountain ranges form natural watershed boundaries because snowmelt and rainwater flow downhill into streams and rivers.

Gulf of Mexico

Colorado

Miss

issip

pi R

iver

Red River

SnakeRiver

Platte River

River

Rio Grande

Arkansas River

River

Missouri

Mississippi

River

River

Oh

io

Colu

mbi

a River

Yellowstone

River

Great Basin

RO

CK

Y

MO

UN

TA

IN

S

Colorado River

Mississippi River

Atlantic Slope

Billings

A P P A

L AC

HI A

N M

OU

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SSouris-Red-Rainy

PacificNorthwest

GreatLakes

California

0

300 miles0

300 km

N

EW

S

Water Resources 423

▶ Watershed Structure Every waterway, whether a large river or a tiny creek, and the land area that drains into it define a watershed. Notice that the Mississippi River watershed is shown in many shades of green on the map. Inside the enormous Mississippi River watershed are several large watersheds of the river’s major tributaries, such as the Ohio River and Missouri River. The Ohio River watershed, shown in lightest green on the map, covers 528,000 square kilometers (204,000 square miles) and parts of 14 states. When the Ohio River drains into the Mississippi River, this large area becomes part of the overall Mississippi River watershed. In fact, each of the watersheds in Figure 3 is made up of many smaller water-sheds. Any river’s watershed can, therefore, be thought of as the region drained by it and all of its tributaries. Not shown on the map are the more than 50 smaller watersheds that make up the Ohio River watershed. The Colorado River watershed has two major parts, an upper basin and a lower basin, shown in shades of orange on the map.

▶ Managing Watersheds The interconnected nature of watersheds greatly influences how they are managed. Look again at the Mississippi River watershed shown in Figure 3. Consider the effect pollution along the Platte River in Nebraska could have on rivers downstream. Once washed into the river, the pollution would flow southeast into the Missouri River. Eventually, it would empty into the Gulf of Mexico. Similarly, the amount of water taken from the Red River in Texas could affect how much is available to people in Louisiana. Clearly, managing water sustainably requires the cooperation of everyone living within the watershed.

FIGURE 3 United States Watersheds This map shows the major watersheds in the continental United States. A watershed consists of all the land that supplies water to a river system. Interpret Maps What forms the eastern boundary of the Colorado River watershed?

FIGURE 4 Protect Your Watershed Signs like this one in Canada help raise awareness about watersheds.

ANSWERS

Figure 3 Rocky Mountains

Well

Water table

Recharge zone

Water table

Zone of aeration

Zone of saturation

Permeable layer

Impermeable layer

WellDry wellDry well

SpringSpring

0133724751a015Ted Smykal08/25/0909/14/0911/06/09

424 Lesson 1

Groundwater Groundwater can be accessed by wells.

Any precipitation reaching Earth’s land surface that does not evaporate, flow into rivers, or get taken up by organisms soaks into the surface. Some of this water trickles downward through the soil to become groundwater, water found below Earth’s surface. Groundwater makes up about one fifth of Earth’s freshwater supply and plays a key role in meeting human water needs.

Aquifers As water is pulled down into the ground by gravity, it encoun-ters different layers of soil and rock. Layers containing spaces, or pores, through which water can pass are called permeable. Layers with few or no pores are called impermeable. Water soaks through permeable layers until it reaches an impermeable layer. Once water reaches an imperme-able layer, it is trapped and can’t move any deeper.

The water then begins to fill up the spaces available in the permeable layers. In this way, groundwater gets contained within aquifers, sponge-like formations of rock, sand, or gravel that hold water. Shown in Figure 5, an aquifer’s upper layer, or zone of aeration, contains pores through which water can flow. In the lower layer, or zone of saturation, these spaces are completely filled with water. The boundary between the two zones is the water table.

FIGURE 5 Aquifers Groundwater is held in aquifers and accessed through wells.

ANSWERS

Reading Checkpoint Sample answer: A long drought may cause a water table to drop.

Lesson 1 Assessment

1. Only 2.5% of Earth’s water is fresh, and 79% of this fresh water is frozen.

2. A watershed includes a river system and the land area that drains into it. It is more effective to manage an entire watershed than a single water source because pol-lution anywhere in the watershed can damage the river system.

3. If a well is overused, the water table might drop below the bot-tom of the well. In this case, you might have to dig the well deeper to reach the zone of saturation.

4. Sample answer: The ability to move water has allowed people to settle in water-scarce areas. As these populations grow, more water is needed, which can result in shortages.

Wyoming South Dakota

TexasMiles

Kilometers

Colorado Kansas

Nebraska

New MexicoOklahoma

0 100

0 160

Saturated thicknessin meters

0–30

30–120

120–240

240–360

Water Resources 425

1. Explain Provide details from the text that support the conclusion that liquid fresh water on Earth is a limited resource.

2. Describe What is a watershed? Why is it more effective to manage an entire watershed as com-pared to a single water source?

3. Relate Cause and Effect Why might you have to dig a well deeper if it is overused?

4. Explore the BIGQUESTION Most civilizations began near a source of fresh water. Over time, however, technology has enabled us to move fresh water great distances. In what ways do you think this kind of technology has contributed to the water shortages many people face today?

1

FIGURE 6 The Ogallala Aquifer The Ogallala aquifer is one of the world’s largest aquifers. It underlies parts of eight U.S. states from South Dakota to Texas.

A water table’s depth is affected by the shape of the land and by the amount of water available. As rock layers rise and dip, so does the water table. The water table rises during times of heavy precipitation or snowmelt and falls with drier weather. Any area where surface water soaks into the ground and reaches an aquifer below is called the recharge zone. Once within the aquifer, the typical rate of horizontal groundwater flow might be only about 1 meter (3.2 feet) per day. The rate varies widely with the perme-ability of the rock and the slope of the aquifer.

Within an aquifer, groundwater may travel horizontally for hundreds of kilometers, remaining underground for a very long time. In fact, the average age of groundwater has been estimated at 1400 years, and some is tens of thousands of years old. This means not only that groundwater is old, but that it can take a long time to recharge once it is depleted.

ReadingCheckpoint

Why might a water table drop?

Getting Groundwater to the Surface How do people access groundwater? Sometimes, the water table naturally rises to the surface creating springs. Other times, groundwater bursts to the surface as a geyser, like Old Faithful in Yellowstone Park. Most of the time, however, we have to dig to gain access to groundwater. A hole dug into an aquifer to reach groundwater is called a well. Wells are dug deep into the zone of saturation so that they won’t dry up during seasonal droughts. But if the water table does drop below the depth of the well, the well will run dry. Dry wells must be dug deeper to keep supplying water to the surface.

Each day in the United States alone, aquifers release 1.9 trillion liters (492 billion gallons) of groundwater through wells, springs, and geysers, into bodies of surface water—nearly as much as the daily flow of the Mississippi River. The world’s largest known aquifer is the Ogallala aquifer, which underlies the Great Plains of the United States (Figure 6). Water from this massive aquifer has enabled American farmers to create the most bountiful grain-producing region in the world.