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Worldwide, we use 70% of the freshwater we withdraw each year from rivers, lakes, and aquifers to irrigate cropland and raise livestock. In arid regions, up to 90% of the regional water supply is used for food production. Industry uses roughly another 20% of the water withdrawn globally each year. Cities and residences use the remaining 10%.

Note: .

Because of global trade, the virtual water used to produce and transport products such as coffee and wheat (also called embedded water) is often withdrawn as groundwater or surface water in another part of the world. Thus, water can be imported in the form of products, often from countries that are short of water.

Large exporters of virtual water—mostly in the form of wheat, corn, soybeans, and other foods—are the European Union, the United States, Canada, Brazil, India, and Australia. Indeed, Brazil’s supply of freshwater per person is more than 8 times the U.S. supply per person, 14 times China’s supply, and 29 times India’s supply. Brazil is becoming one of the world’s largest exporters of virtual water. However, prolonged severe droughts in parts of Australia, the United States, and the European Union are stressing the abilities of these countries to meet the growing global demand for their food exports.

· 13.1dWater Use Is Increasing

· According to hydrologists, scientists who study water and its properties and movement, two-thirds of the annual surface runoff of freshwater into rivers and streams is lost in seasonal floods and is not available for human use. The remaining one-third is reliable surface runoff—defined as the portion of runoff that is regarded as a stable source of freshwater from year to year. GREEN CAREER: Hydrologist

· Since 1900, the human population tripled, global water withdrawals increased sevenfold, and per capita water withdrawals quadrupled. As a result, we now withdraw an estimated 34% of the world’s reliable runoff. This is a global average. In the arid American southwest, up to 70% of the reliable runoff is withdrawn for human purposes, mostly for irrigation. Some water experts project that because of population growth, rising rates of water use per person, longer dry periods in some areas, and unnecessary water waste, we are likely to be withdrawing up to 90% of the world’s reliable freshwater runoff by 2025.

· Worldwide, we use 70% of the freshwater we withdraw each year from rivers, lakes, and aquifers to irrigate cropland and raise livestock. In arid regions, up to 90% of the regional water supply is used for food production. Industry uses roughly another 20% of the water withdrawn globally each year. Cities and residences use the remaining 10%.

· Your water footprint is a rough measure of the volume of freshwater that you use directly or indirectly. Your daily water footprint includes the freshwater you use directly (for example, to drink, bathe, or flush a toilet) and the water you use indirectly through the food, energy, and products you consume. (See the Case Study that follows for information on U.S. water use.) The three largest water footprints in the world belong to India, the United States, and China, in that order.

· Case Study

· Freshwater Resources in the United States

· According to the U.S. Geological Survey (USGS), the major uses of groundwater and surface freshwater in the United States are the cooling of electric power plants, irrigation, public water supplies, industry, and livestock production (Figure 13.5, left). The average American directly uses about 370 liters (98 gallons) of freshwater a day—enough water to fill 2.5 typical bathtubs. (The average bathtub can contain about 151 liters or 40 gallons of water.) Household water is used mostly for flushing toilets, washing clothes, taking showers, and running faucets, or is lost through leaking pipes, faucets, and other fixtures (Figure 13.5, right).

· Figure 13.5

· Comparison of primary uses of water in the United States (left) and uses of water in a typical U.S. household (right).

· Data Analysis:

· In the right-hand chart, which three categories, added together, are smaller than the amount of water lost in leaks?

·

· (Compiled by the authors using data from U.S. Geological Survey, World Resources Institute, and American Water Works Association.)

· The United States has more than enough renewable freshwater to meet its needs. However, it is unevenly distributed and much of it is contaminated by agricultural and industrial practices. The eastern states usually have ample precipitation, whereas many western and southwestern states have little (Figure 13.6).

· Figure 13.6

· Long-term average annual precipitation and major rivers in the continental United States.

·

· (Compiled by the authors using data from U.S. Water Resources Council and U.S. Geological Survey.)

· In the eastern United States, most water is used for manufacturing and for cooling power plants (with most of the water heated and returned to its source). In many parts of this area, the most serious water problems are flooding, occasional water shortages because of drought, and pollution.

· In the arid and semiarid regions of the western half of the United States (Core Case Study), irrigation counts for as much as 85% of freshwater use. Much of it is lost to evaporation and a great deal of it is used to grow crops that require a lot of water. The major water problem is a shortage of freshwater runoff caused by low precipitation (Figure 13.6), high evaporation, and recurring prolonged drought.

· Groundwater is one of the most precious of all U.S. resources. About half of all Americans (and 95% of all rural residents) rely on it for drinking water. It makes up about half of all irrigation water, feeds about 40% of the country’s streams and rivers, and provides about one-third of the water used by U.S. industries.

· Water tables in many water-short areas, especially in the dry western states, are dropping as farmers and rapidly growing urban areas draw down many aquifers faster than they can be recharged. The U.S. Department of the Interior has mapped out water scarcity hotspots in 17 western states (Figure 13.7). In these areas, there is competition for scarce freshwater to support growing urban areas, irrigation, recreation, and wildlife. This competition for freshwater could trigger intense political and legal conflicts between states and between rural and urban areas within states. In addition, Columbia University climate researchers led by Richard Seager used well-tested climate models to project that the southwestern United States is very likely to have long periods of extreme drought throughout most of the rest of this century.

· Figure 13.7

· Water scarcity hotspots in 17 western states that, by 2025, could face intense conflicts over scarce water needed for urban growth, irrigation, recreation, and wildlife.

· Question:

· Which, if any, of these areas are found in the Colorado River basin (Core Case Study)?

·

· (Compiled by the authors using data from U.S. Department of the Interior and U.S. Geological Survey.)

· The Colorado River system (Figure 13.1) is directly affected by such drought. There are three major problems associated with the use of freshwater from this river (Core Case Study). First, the Colorado River basin includes some of the driest lands in the United States and Mexico. Second, long-standing legal agreements between Mexico and the affected western states allocated more freshwater for human use than the river can supply, even in rare years when there is no drought. These pacts allocated no water for protecting aquatic and terrestrial wildlife. Third, since 1960, because of drought, damming, and heavy withdrawals, the river has rarely flowed all the way to the Gulf of California and this has degraded the river’s aquatic ecosystems and dried up its delta (which we discuss later in this chapter).

· Freshwater that is not directly consumed but is used to produce food and other products is called virtual water. It makes up a large part of the water footprints of individuals, especially in more-developed countries. Agriculture accounts for the largest share of humanity’s water footprint. Producing and delivering a typical quarter-pound hamburger, for example, takes about 2,400 liters (630 gallons or 16 bathtubs) of freshwater—most of it used to grow grain to feed cattle. Producing a smart phone requires about 910 liters (240 gallons or 6 bathtubs) of freshwater. Figure 13.4 shows one way to measure the amounts of virtual water used for producing and delivering products. These values can vary depending on how much of the supply chain is included, but they give us a rough estimate of the size of our water footprints.

· Figure 13.4

· Producing and delivering a single one of each of the products listed here requires the equivalent of nearly one and usually many bathtubs full of freshwater, called virtual water.

·

·

· (Compiled by the authors using data from UN Food and Agriculture Organization, UNESCO-IHE Institute for Water Education, World Water Council, and Water Footprint Network.); Bathtub: Baloncici/ Shutterstock.com. Coffee: Aleksandra Nadeina/ Shutterstock.com. Bread: Alexander Kalina/ Shutterstock.com. Hamburger: Joe Belanger/ Shutterstock.com. T-shirt: grmarc/ Shutterstock.com. Jeans: Eyes wide/ Shutterstock.com. Car: L Barnwell/ Shutterstock.com. House: Rafal Olechowski/ Shutterstock.com

· Note: .

· Because of global trade, the virtual water used to produce and transport products such as coffee and wheat (also called embedded water) is often withdrawn as groundwater or surface water in another part of the world. Thus, water can be imported in the form of products, often from countries that are short of water.

· Large exporters of virtual water—mostly in the form of wheat, corn, soybeans, and other foods—are the European Union, the United States, Canada, Brazil, India, and Australia. Indeed, Brazil’s supply of freshwater per person is more than 8 times the U.S. supply per person, 14 times China’s supply, and 29 times India’s supply. Brazil is becoming one of the world’s largest exporters of virtual water. However, prolonged severe droughts in parts of Australia, the United States, and the European Union are stressing the abilities of these countries to meet the growing global demand for their food exports.

13.2aAquifer Depletion

· 13.3a

· Large Dams

· A dam is a structure built across a river to control its flow. Usually, dammed water creates an artificial lake, or reservoir, behind the dam (see chapter-opening photo). The purpose of a dam-and-reservoir system is to capture and store the surface runoff from a river’s watershed, and release it as needed to control floods, to generate electricity (hydropower), and to supply freshwater for irrigation and for towns and cities. Reservoirs also provide recreational activities such as swimming, fishing, and boating. Large dams and reservoirs provide benefits but also have drawbacks (Figure 13.15).

·

· Figure 13.15

· Trade-offs: Large dam-and-reservoir systems have advantages (green) and disadvantages (orange).

·

· Critical Thinking:

· Do the advantages outweigh the disadvantages? Why or why not?

·

· An illustration shows a dam site with hydropower generation, on a three-dimensional surface, where the waters from the river is blocked by a wall with sluice openings through which the excess water flows outside. Hydropower generators looking like rods are shown on the walls over the sluice openings and the electricity generated is taken through wires and connected to a transformer shown behind. Vegetation is found on the banks of the river and pure water is tapped in a reservoir on the banks of the river. Ten callouts are shown in the illustration. The first callout on the left side pointing to the arrangement for irrigation reads, “Provides irrigation water above and below the dam.” The second callout pointing to the reservoir reads, “Provides water for drinking.” The third callout pointing to the dam water reads, “Reservoir useful for recreation and fishing.” The fourth callout pointing to the hydropower generators reads, “Can produce cheap electricity (hydropower).” The fifth callout pointing to the water coming out of the sluice openings reads, “Reduces down-stream flooding of cities and farms.” The sixth callout pointing to the road near the dam water on the right side top reads, “Flooded land destroys forests or cropland and displaces people.” The seventh callout pointing to the dam water reads, “Large losses of water through evaporation.” The eighth callout pointing to the reservoir reads, “Deprives downstream cropland and estuaries of nutrient-rich silt.” The ninth callout pointing to the walls that block the water from the reservoir reads, “Risk of failure and devastating downstream flooding.” The tenth callout pointing to the water flowing through the sluice opening reads, “Disrupts migration and spawning of some fish.” Another insight of the illustration is shown below which shows the reservoir and the dam which are labeled. The intake is indicated by an arrow mark from the reservoir and the water passes through Turbine like a ‘Plus’ symbol and labeled, and then flows outside. Powerhouse is shown just above the turbine and the generated power is taken through power lines connected to transformer.Enlarge Image

· The world’s 45,000 large dams capture and store about 14% of the world’s surface runoff. They provide water for almost half of all irrigated cropland and supply more than half of the electricity used in 65 countries. Dams have increased the annual reliable runoff available for human use by nearly 33%. As a result, the world’s reservoirs now hold 3–6 times more freshwater than the total amount flowing at any moment in all of the world’s natural rivers.

·

· However, dam-and-reservoir systems have drawbacks. For example, the world’s reservoirs have displaced 40-80 million people from their homes and flooded large areas of mostly productive land. Dams have also impaired some of the important ecosystem services that rivers provide (see Figure 8.14, left).

·

· Another effect of dams, along with water withdrawals, is a reduction in the flow of rivers. According to a study by the World Wildlife Fund (WWF) only 21 of the planet’s 177 longest rivers consistently run all the way to the sea before running dry. As a result, aquatic habitat along rivers and at their mouths has been severely degraded (see Case Study that follows). Another effect of diminished river flows, according to the WWF study, is that about one out of five of the world’s freshwater fish and plant species are either extinct or endangered. This helps explain why estimated extinction rates for freshwater life are four to six times larger than for marine or terrestrial species.

·

· Case Study

· How Dams Can Kill a Delta

· Since 1905, the amount of water flowing to the mouth of the Colorado River (Core Case Study) has dropped dramatically. In most years since 1960, the river has barely reached the Gulf of California (Figure 13.16).

·

· Figure 13.16

· The measured flow of the Colorado River (Core Case Study) at its mouth has dropped sharply since 1905 because of multiple dams, water withdrawals for agriculture and urban areas, and prolonged drought.

·

· A graphical representation shows the flow of Colorado River. It is represented with year starting with 1910 and ending with 2000 with an interval of 10 years along the x-axis and flow in (billion cubic meters) starting from 0 and ending at 35 with an interval of 5 along the y-axis. The graph is shaky with several peaks and downfalls. The curve starts from 1910 along the x-axis and 24 along the y-axis and shows several ups and downs and reaches a height of 1935 along the x-axis and above 15 along the y-axis and labeled as, “Hoover dam completed 1935.” The graph again shows ups and downs and touches the x-axis just before 1960 and shows slight rise and then slopes down again and touches x-axis at 1963 and labeled above as, “Glen Canyon dam completed 1963.” Then the graph continues over the x-axis until 1980 and then shows ups and downs till it reaches 2000 along the x-axis. In between there is light peak that reaches 15 along the y-axis and above 1980 on x-axis. The curve touches the x-axis again at the end.

· (Compiled by the authors using data from the U.S. Geological Survey)

· The Colorado River once emptied into a vast delta, with forests, lagoons, and marshes rich in plant and animal life. The delta also supported a thriving coastal fishery for hundreds of years. Since the damming of the Colorado River—within one human lifetime—this biologically diverse delta ecosystem has collapsed and is now covered mostly by mud flats and desert.

·

· Historically, about 80% of the water withdrawn from the Colorado has been used to irrigate crops and raise cattle. That is because the U.S. government (taxpayers) paid for the dams and reservoirs and have supplied many farmers and ranchers with water at low prices. These subsidies have led to wasteful use of irrigation water for growing thirsty crops such as rice, cotton, almonds, and alfalfa in dry areas.

·

· In 2014, the floodgates of the Morelos Dam near Yuma, Arizona, were opened for 2 months to send Colorado River water through the delta to the Gulf of California for the first time in years. Researchers are evaluating the effects of this experiment, but short-term results were dramatic, according to National Geographic Fellow and water policy expert Sandra Postel. Thousands of trees began to grow along the river’s banks and groundwater in the delta area was partially recharged for the first time in many years.

·

·

·

· Water experts urge the seven states using the Colorado River to enact and enforce strict water conservation measures. They also call for sharply decreasing or phasing out state and federal government subsidies for agriculture in this region. The goal would be to shift water-thirsty crops to less arid areas and severely restrict the watering of golf courses and lawns in the desert areas of the Colorado River basin. They suggest that the best way to implement such solutions is to sharply raise the historically low price of the river’s freshwater over the next decade—another application of the full-cost pricing principle of sustainability.

·

· In 2019, the seven western states sharing the Colorado River agreed on a plan to voluntarily cut their water use from the river to prevent the federal government from imposing mandatory levels on the amount of water they could withdraw from the river. This is an important step but much more needs to be done according to water experts, as discussed above.

·

· Critical Thinking

· What are three steps you would take to deal with the problems of the Colorado River system?

·

· Reservoirs also have limited lifespans. Within 50 years, reservoirs behind dams typically fill up with sediments (mud and silt), which make them useless for storing water or producing electricity. In the Colorado River system (Core Case Study), the equivalent of roughly 20,000 dump-truck loads of silt are deposited on the bottoms of the Lake Powell and Lake Mead reservoirs every day. Sometime during this century, these two vital reservoirs will very likely be too full of silt to function as designed. About 85% of all U.S. dams will be 50 years old or older by 2025. Some aging dams have been removed because their reservoirs have filled with silt.

·

· If climate change occurs as projected during this century, water shortages will intensify in many parts of the world. For example, mountain snows that feed the Colorado River system will melt faster and earlier, making less freshwater available to the river system when it is needed for irrigation during hot and dry summer months. Agricultural production would drop sharply and the region’s major desert cities such as Las Vegas, Nevada and Phoenix, Arizona, would be challenged to survive.

·

· If some of the Colorado River’s largest reservoirs keep dropping dramatically or become filled with silt during this century, the region will experience costly water and economic disruptions. For example, by 2013, the water level in Lake Mead had dropped below the Hoover Dam’s intake pipes. The city of Las Vegas has spent more than $800 million to build lower intake pipes in order to maintain hydroelectric production.

·

· Dams can also fail and flood downstream areas. According to the U.S. Army Corps of Engineers, 30% of the more than 90,000 dams in the United States are at risk of failure and need repairs.

·

· Nearly 3 billion people in South America, China, India, and other parts of Asia—that is, nearly half the world’s population—depend on river flows fed by mountain glaciers. The glaciers serve as freshwater savings accounts. They store ice and snow in wet periods and release it slowly during dry seasons for use on farms and in cities. In 2015, according to the World Glacier Monitoring Service, many of these mountain glaciers have been shrinking for 24 consecutive years, mostly due to a warming atmosphere.

·

· 13.3b

· Removing Salt from Seawater to Provide Freshwater

· Desalination is the process of removing dissolved salts from ocean water or from brackish (slightly salty) water in aquifers or lakes. It is another way to increase supplies of freshwater.

·

· Currently, the two most widely used methods for desalinating water are distillation and reverse osmosis (Figure 13.17). Distillation involves heating saltwater until it evaporates (leaving behind salts in solid form) and condenses as freshwater. Reverse osmosis (or microfiltration) uses high pressure to force saltwater through a membrane filter with pores small enough to remove the salt and other impurities. Reverse osmosis requires less than a third of the energy needed to distill saltwater.

·

· Figure 13.17

· Desalination: Reverse osmosis (left) involves applying high pressure (a) to force sea water from one chamber into another through a semipermeable membrane (b) that separates the salt (c) producing freshwater (d). Distillation (right) involves heating seawater (a) to produce steam (b) which is then condensed (c) and collected as freshwater (d) while brine is also collected (e) for processing.

·

· An illustration shows two images. The first illustration shows reverse osmosis which shows two chambers separated by a semipermeable membrane. The sea water in one of the chambers on the left side is given high pressure from upward, indicated by arrow marks pointing downward and labeled as a. The sea water is forced through the semi-permeable membrane which is indicated through the arrow marks and labeled as b. Fresh water production on the left, is labeled as c and indicated by arrow mark with opening. The level of water in the next chamber is low and process is continued which is distillation indicated as d with an opening chamber on the right side. The second illustration shows distillation which shows two chambers in a container. In the right chamber, the water is heated which is indicated by a circular rod-like structure in red and labeled as a. The steam generated is condensed which is indicated by a spiral rod moving from the heated water towards upward and then to the next chamber and labeled as b, where the condensation starts above the heating process. Then the water droplets are shown in the next chamber, on left which is the fresh water and labeled as c. The brine which is collected is indicated with an outlet of the chamber is labeled as d. The outlet of the hot chamber shows an arrow mark pointing outward on the right side near the heating rod and labeled as e.Enlarge Image

· The world’s more than 18,400 desalination plants, including more than 320 in the United States, supply less than 1% of the freshwater used in the United States and in the world.

·

· Three major problems hinder the widespread use of desalination.

·

· It is costly because removing salt from seawater requires a lot of energy.

·

· Pumping large volumes of seawater through pipes requires chemicals to sterilize the water and prevent algae growth. This kills many marine organisms and requires large inputs of energy and money.

·

· Desalination produces huge quantities of wastewater that are much saltier than ocean water and that require proper disposal. Dumping the salty waste into coastal ocean waters increases the salinity of those waters, which can threaten food resources and aquatic life. Disposing the salty wastes on land can contaminate groundwater and surface water.

·

· Currently, desalination is practical only for water-short countries and cities that can afford its high cost. However, scientists and engineers are working to develop better and more affordable desalination technologies. (Science Focus 13.2)

·

· Science Focus 13.2

· The Search for Better Desalination Technology

· Reverse osmosis (Figure 13.17, left) is the favored desalination technology because it requires much less energy than distillation, but it is still energy intensive. Much of the scientific research in this field is aimed at improving the membrane and the pre- and post-treatment processes to make desalination more energy efficient. Scientists are working to develop new, more efficient and affordable membranes that can separate freshwater from saltwater under lower pressure, which would require less energy.

·

· One promising material that might serve this purpose is one-atom-thick graphene. Such technological advances have brought the cost of desalination down, but not enough yet to make it affordable or useful for large-scale irrigation or to meet much of the world’s demand for drinking water.

·

·

·

· A team of scientists at the Massachusetts Institute of Technology (MIT), led by Martin Z. Bazant, is evaluating the use of an electric shock to separate saltwater and freshwater. Scientists are also considering ways to use solar and wind energy—applying one of the three scientific principles of sustainability—in combination with conventional power sources to help bring down the cost of desalinating seawater. Saudi Arabia has the world’s largest solar-powered desalination project. It uses concentrated solar energy to power new filtration technology at a plant that will meet the daily water needs of 100,000 people. Two Australian companies, Energetech and H2AU, have joined forces to build an experimental desalination plant that uses the power generated by ocean waves to drive reverse-osmosis desalination. This approach produces no air pollution and uses renewable energy.

·

· Some scientists argue for building fleets of such floating desalination plants. They could operate out of sight from coastal areas and transfer the water to shore through seabed pipelines or in food-grade shuttle tankers. Because of their distance from shore, the ships could draw water from depths below where most marine organisms are found. The resulting brine could be returned to the ocean and diluted far away from coastal waters.

·

· These methods would cut the costs of desalination, but they would still be high. Analysts expect desalination to be used more widely in the future, as water shortages become worse. However, there is still a lot of research to do before desalination can become an affordable major source of freshwater. GREEN CAREER: Desalination engineer

·

· Critical Thinking

· Do you think that improvements in desalination will justify highly inefficient uses of water, such as maintaining swimming pools, fountains, and golf courses in desert areas? Why or why not?

·

· Learning from Nature

· Scientists are trying to develop more efficient and affordable ways to desalinate seawater by mimicking how our kidneys take salt out of water and how fish in the sea survive in saltwater. 13.4a

· Water Transfers Have Benefits and Drawbacks

· In some heavily populated dry areas of the world, governments have tried to solve water shortage problems by using canals and pipelines to transfer water from water-rich areas to water-poor areas. Much of this water is transferred for irrigating crops. For example, if you eat lettuce in the United States, chances are it was grown in the arid Central Valley of California, partly with the use of irrigation water from snow melting off the tops of the High Sierra Mountains of northeastern California.

·

· The California State Water Project (Figure 13.18) is one of the world’s largest freshwater transfer projects. It uses a maze of giant dams, pumps, and lined canals, or aqueducts (photo in Figure 13.18), to transfer freshwater from the mountains of northern California to heavily populated cities and agricultural regions of water-poor central and southern California.

·

· Figure 13.18

· The California State Water Project transfers huge volumes of freshwater from one watershed to another. The arrows on the map show the general direction of water flow. The photo shows one of the aqueducts carrying water within the system.

·

· Critical Thinking:

· What effects might this system have on the areas from which the water is taken?

·

· A map shows California state water project, where California, Shasta lake, Sacramento river, Sacramento, lake Tahoe, Oroville dam and reservoir, San Luis dam and reservoir, Sierra mountain range, San Joaquin river, Fresno, Santa Barbara, Los Angeles, San Diego, Nevada, San Francisco, and Hoover dam and reservoir are labeled. The Colorado River, the Colorado River aqueduct, North Bay aqueduct, south bay aqueduct, California aqueduct, and Los Angeles aqueduct are shaded with a red line and labeled. A photo on the top of the map, shows a river and dry land on both sides of the river, along with the bridge over it.Enlarge Image

· Sarahleen/National Geographic Image Collection

· This massive water transfer has yielded many benefits. California’s heavily irrigated Central Valley supplies half of the nation’s fruits and vegetables, and the arid cities of San Diego and Los Angeles have grown and flourished because of the water transfer.

·

· However, water transfers can also have high environmental, economic, and social costs (see Case Study that follows). They usually involve large water losses, through evaporation and leaks in the water-transfer systems. They also degrade ecosystems in areas from which the water is taken. For example, the Chinese government is implementing its South–North Water Diversion Project to transfer water from the Yangtze River in southern China to the thirsty north. China’s massive water transfer will displace more than 350,000 villagers who will have to move from lands they have farmed for generations. In addition, scientists warn that removing huge volumes of water from the Yangtze River could severely damage its ecosystem, which has been suffering from its worst drought in 50 years.

·

· Case Study

· The Aral Sea Disaster: An Example of Unintended Consequences

· The shrinking of the Aral Sea (Figure 13.19) is the result of a water transfer project in central Asia. Starting in 1960, enormous amounts of irrigation water were diverted from the two rivers that supply water to the Aral Sea. The goal was to create one of the world’s largest irrigated areas, mostly for raising cotton and rice. The irrigation canal, the world’s longest, stretches more than 1,300 kilometers (800 miles).

·

· Figure 13.19

· Natural capital degradation: The Aral Sea, straddling the borders of Kazakhstan and Uzbekistan, was one of the world’s largest saline lakes. These satellite photos show the sea in 1976 (left) and in 2016 (right). The shrinkage of the southern Aral Sea has continued since 2016.

·

· Critical Thinking:

· What do you think should be done to help prevent further shrinkage of the Aral Sea?

·

· A figure shows two photos. The first photo shows the complete Aral Sea which was in existence during 1976 on the left side. The second photo shows the shrunken Aral Sea which shows only a few portions of the Aral Sea which was in existence during 2015 on the right side.Enlarge Image

· WorldSat International, Inc. All rights reserved; NASA

· This project, coupled with drought and high evaporation rates due to the area’s hot and dry climate, has caused a regional ecological and economic disaster. Since 1961, the sea’s salinity has risen sevenfold and the average level of its water has dropped by an amount roughly equal to the height of a six-story building. The Southern Aral Sea has lost 90% of its volume of water and most of its lake bottom is now a white salt desert (Figure 13.19, right photo). Water withdrawals have reduced the two rivers feeding the sea to mere trickles.

·

· About 85% of the area’s wetlands have been eliminated and about half the local bird and mammal species have disappeared. The sea’s greatly increased salt concentration—three times saltier than ocean water—has caused the presumed local extinction of 26 of the area’s 32 native fish species. This has devastated the area’s fishing industry, which once provided work for more than 60,000 people. Fishing villages and boats once located on the sea’s coastline now sit abandoned in a salty desert.

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· Winds pick up the sand and salty dust and blow it onto fields as far as 500 kilometers (310 miles) away. As the salt spreads, it pollutes water and kills wildlife, crops, and other vegetation. Aral Sea dust settling on glaciers in the Himalayas is causing them to melt at a faster-than-normal rate.

·

· The shrinkage of the Aral Sea has also altered the area’s climate. The shrunken sea no longer acts as a thermal buffer to moderate the heat of summer and the extreme cold of winter. There is less rain, summers are hotter and drier, winters are colder, and the growing season is shorter. The combination of such climate change and severe salinization has reduced crop yields by 20–50% on almost one-third of the area’s cropland—the opposite of the project’s intended effects.

·

· Since 1999, the UN, the World Bank, and the five countries surrounding the lake have worked to improve irrigation efficiency. They have also partially replaced thirsty crops with other crops that require less irrigation water. Because of a dike built to block the flow of water from the Northern Aral Sea into the southern sea, the level of the northern sea has risen by 2 meters (7 feet), its salinity has dropped, dissolved oxygen levels are up, and it supports a healthy fishery. However, the formerly much larger southern sea is still shrinking and is likely to dry up completely within a few years.

·

· Other large lakes are also drying up. They include Bolivia’s Lake Poopó, Iran’s Lake Urmia, and Lake Chad located in several West African countries.

·

· In California, sending water south has reduced the flow of the Sacramento River, threatening fisheries and reducing the flushing action that helps to cleanse the San Francisco Bay of pollutants. As a result, the bay has suffered from pollution, and the flow of freshwater to its coastal marshes and other ecosystems has dropped. These factors have placed stress on wildlife species that depend on the bay’s many ecosystems. Water was also diverted from streams that flow into Mono Lake, an important feeding stop for migratory birds. This lake experienced an 11-meter (35-foot) drop in its water level before the diversions were stopped. For a while, the lake’s entire ecosystem was in jeopardy.

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· The federal government and the state of California subsidized this water transfer project. These subsides have promoted inefficient use of large volumes of water to irrigate thirsty crops such as lettuce, alfalfa, and almonds in desert-like areas. In central California, agriculture consumes three-fourths of the water that is transferred. Much of this water is wasted by inefficient irrigation systems. Studies show that making irrigation just 10% more efficient would provide all the water needed for domestic and industrial uses in southern California.

·

According to several studies, climate change during this century will make matters worse in California by reducing surface water availability. California depends on snowpacks, bodies of densely packed, slowly melting snow in the High Sierra Mountains, for more than 60% of its freshwater during summer according to the Sierra Nevada Conservancy. Projected atmospheric warming could shrink the snowpacks by as much as 40% by 2050 and by as much as 90% by the end of this century. This will sharply reduce the amount of freshwater available for northern California residents and ecosystems, as well for the transfer of water to central and southern California. 13.5aCutting Water Waste

According to water resource expert Mohamed El-Ashry of the World Resources Institute, about 66% of the freshwater used in the world and about 50% of the freshwater used in the United States is lost through evaporation, leaks, and inefficient use. El-Ashry estimates that it is economically and technically feasible to reduce such losses to 15%, thereby meeting most of the world’s future freshwater needs.

Why do we have such large losses of freshwater? According to water resource experts, there are two major reasons. First, the cost of freshwater to most users is low due mostly to government subsidies—a violation of the full-cost pricing principle of sustainability. This gives users little or no financial incentive to invest in water-saving technologies.

Higher prices for freshwater encourage water conservation but make it difficult for low-income farmers and city dwellers to buy enough water to meet their needs. When South Africa raised water prices, it dealt with this problem by establishing lifeline rates, which give each household a set amount of free or low-priced water to meet basic needs. When users exceed this amount, they pay increasingly higher prices as their water use increases. This is a user-pays approach.

The second major cause of unnecessary waste of freshwater is a lack of government subsidies for improving the efficiency of water use. Many hydrologists and economists call for replacing current water subsidies that encourage water waste with subsidies that would encourage using water more efficiently. Understandably, farmers and industries that receive subsidies that keep water prices low have vigorously opposed efforts to eliminate or reduce them.

· 13.5b

· Improving Irrigation Efficiency

·

·

· Since 1980, the amount of food that can be grown per drop of water has roughly doubled. In addition, since the 1970s, the amount of water used per person in the United States has dropped by about 33%, after rising for decades. Most of these water savings have come from improvements to irrigation efficiency in the United States and other more-developed countries.

·

· However, there is still a long way to go, especially in less-developed countries. Only about 60% of the world’s irrigation water reaches crops, which means that most irrigation systems are highly inefficient. The least inefficient irrigation is flood irrigation, in which water is pumped from a groundwater or surface water source through unlined ditches where it flows by gravity to the crops being watered (Figure 13.20, left). This method delivers far more water than is needed for crop growth, and typically, about 45% of it is lost through evaporation, seepage, and runoff.

·

· Figure 13.20

· Traditional irrigation methods rely on gravity and flowing water (left). Newer systems such as center-pivot, low-pressure sprinkler irrigation (right) and drip irrigation (center) are far more efficient.

·

· An illustration shows three sections. The first section shows the crops or plants planted on the soil in rows which is shown on a three-dimensional surface and the text below reads, “Gravity Flow (Efficiency 60% and 80% with surge values), Water usually comes from an aqueduct system or a nearby river.” The second section shows the trees being planted in rows and columns and for each tree a circular rod-like structure is placed around and the circular rods are interconnected by horizontal and vertical rods forming a newer system of irrigation called, “drip irrigation.” The text below reads, “Drip Irrigation (efficiency 90-95%), above or below ground pipes or tubes deliver water to individual plant roots.” The third section shows the crops being planted in a circular pattern on a three-dimensional surface and irrigated through pipes from the top which are supported by the pivot at the center. The insight of this illustration is shown above, which is indicated by an arrow mark from the center pivot, shows a plane surface where the crops are planted and the water is sprinkled from above through pipes on each row, from the rod structure which is held in a horizontal position through a vertical rod support. The text below reads, “Centre pivot (efficiency 80% with low pressure sprinkler and 90-95% LEPA sprinkler), Water usually pumped from underground and sprayed from mobile boom with sprinklers.”Enlarge Image

· Another inefficient system is the traditional spray irrigation system, a widely used tool of industrialized crop production. It sprays huge volumes of water onto large fields, and as much as 40% of this water is lost to evaporation, especially in dry and windy areas, according to the U.S. Geological Survey (USGS). These systems are commonly used in the Midwestern United States and have helped to draw down the Ogallala Aquifer (Case Study, this chapter).

·

· More efficient irrigation technologies greatly reduce water losses by delivering water more precisely to crops—a more crop per drop strategy. For example, a center-pivot, low-pressure sprinkler (Figure 13.20, right), which uses pumps to spray water on a crop, allows about 80% of the water to reach crops. An improved center-pivot system that sprays the water closer to the ground puts 90–95% of the water where crops need it.

·

· Drip, or trickle irrigation (Figure 13.20, center), is the most efficient way to deliver small amounts of water precisely to crops. It consists of a network of perforated plastic tubing installed at or below the ground level. Small pinholes in the tubing deliver drops of water at a slow and steady rate, close to the roots of individual plants and 90–95% of the water to reaches the crops.

·

· Drip irrigation is used on less than 5% of the irrigated crop fields in the world and in the United States, largely because most drip irrigation systems are costly. This percentage rises to 13% in the U.S. state of California, 66% in Israel, and 90% in Cyprus. If freshwater were priced closer to the value of the ecosystem services it provides, and if government subsidies for inefficient use of water were reduced or eliminated, drip irrigation could be used to irrigate most of the world’s crops. For farmers receiving irrigation water subsidies, it is cheaper to waste water than to conserve it.

·

· According to the UN, reducing the current global withdrawal of water for irrigation by just 10% would save enough water to grow crops and meet the estimated additional water demands of the earth’s cities and industries through 2025. This would also reduce the need for costly desalination.

3.5c

Conserving Water through Low-Tech Methods

Many of the world’s poor farmers use low-cost, traditional irrigation technologies that are more sustainable than most large-scale irrigation systems. For example, millions of farmers in Bangladesh and other countries where water tables are high use human-powered treadle pumps to bring groundwater up to the earth’s surface and into irrigation ditches (Figure 13.21). These wooden devices are inexpensive and easy to build from local materials. One such pump developed by the nonprofit International Development Enterprises (IDE) uses 60–70% less water than a conventional gravity-flow system to irrigate the same amount of cropland at one-tenth the cost of conventional drip systems.

Figure 13.21

Solutions: In areas of Bangladesh and India, where water tables are high, many small-scale farmers use treadle pumps to supply irrigation water to their fields.

A photo shows a field in which men, and women are working and on the left corner, a treadle pump with a girl standing on it and the pump’s water supply is shown.

Courtesy of International Development Enterprises

Other farmers in some less-developed countries use buckets, small tanks with holes, or simple plastic tubing systems for drip irrigation. One ingenious system makes use of solar energy to drive drip irrigation (Individuals Matter 13.1).

Individuals Matter 13.1

Jennifer Burney: Environmental Scientist and National Geographic Explorer

A photo shows Jennifer Burney, who is an Environmental Scientist and National Geographic Explorer.

UC San Diego/National Geographic Image Collection

Environmental scientist and National Geographic Explorer Jennifer Burney notes that subsistence farmers represent the majority of the world’s poorest people and need to boost their productivity for better standards of living and health. She is trying to help such farmers in Africa to grow, distribute, and cook their food using resources such as water, fertilizer, and energy as efficiently as possible. She also helps them avoid unsustainable practices such as wasteful irrigation and fertilizer runoff that are the legacy of large-scale industrial farming in the developed world.

For example, in arid sub-Saharan Africa, farmers must depend on rainfall for raising crops on small plots because only 20% of the rainfall flows into streams and aquifers while the rest evaporates. Overpumping can quickly deplete the groundwater. These factors, worsened by drought, make it hard for farmers to feed their families.

To deal with this problem, Burney has helped farmers to connect two technologies—solar energy systems and drip irrigation. Drip irrigation systems sip water and drip it directly onto plant roots instead of pumping and dumping it. Solar-powered pumps work without the need for batteries or fuel. On sunny days, when crops need water more, the solar panels speed the pumping; on cloudy days when there is less evaporation, the pumping slows down. Thus, only the amount of water that is needed is pumped on most days. This has allowed farmers to grow fruits and vegetables on a larger scale and to improve their incomes and food security.

Rainwater harvesting is another simple and inexpensive way to provide water. It involves using pipes from rooftops and channels dug in the ground to direct rainwater that would otherwise run off the land. It can be stored in underground or aboveground storage tanks (cisterns), ponds, and plastic barrels for use during dry seasons. This is especially useful in countries such as India, where much of the rain comes in a short monsoon season.

In dry mountainous coastal areas, such as in Peru, some communities are capturing water from fog that rolls in off the ocean on most days. On the seaward hillsides, they erect large flat nets on which the fog condenses. The resulting water drops roll off the nets into troughs that channel the water into holding tanks.

Other strategies used by poor farmers to increase the amount of crop per drop of rainfall include polyculture farming to create more canopy cover and reduce evaporative water losses; planting deep-rooted perennial crop varieties (see Figure 12.D); controlling weeds; and mulching fields to retain more moisture.

Figure 13.22 summarizes several ways to reduce water losses in crop irrigation. Since 1950, Israel has used many of these techniques to slash irrigation water losses by 84% while irrigating 44% more land. Israel now treats and reuses 30% of its municipal sewage water for crop production and plans to increase this to 80% by 2025. Israel also uses desalination to provide nearly half of its water. The government also gradually eliminated most water subsidies to raise Israel’s price of irrigation water, which is now one of the highest in the world.

Figure 13.22

Ways to reduce freshwater losses in irrigation.

Critical Thinking:

Which two of these solutions do you think are the best ones? Why?

An illustration provides information about Reducing Irrigation Water Losses in bullet points and the text below reads, “Avoid growing thirsty crops in dry areas, import water-intensive crops and meat, encourage organic farming and polyculture to retain soil moisture, monitor soil moisture to add water only when necessary, expand use of drip irrigation and other efficient methods, irrigate at night to reduce evaporation, line canals that bring water to irrigation diches, and irrigate with treated wastewater.”

13.5dCutting Freshwater Waste in Industries and Homes

Producers of chemicals, paper, oil, coal, primary metals, and processed foods consume almost 90% of the freshwater used by industries in the United States. The pulp and paper industry uses more water to produce a ton of product than any other industry. Some of these industries recapture, purify, and recycle water to reduce their water use and water treatment costs. For example, more than 95% of the water used to make steel can be recycled. Even so, most industrial processes could be redesigned to use much less water. GREEN CAREER: Water conservation specialist

Flushing toilets with freshwater—most of it clean enough to drink—is the single largest use of domestic freshwater in the United States and accounts for about one-fourth of home water use. Since 1992, U.S. government standards have required that new toilets use no more than 6.1 liters (1.6 gallons) of water per flush. Even at this rate, just two flushes of such a toilet use more than the daily amount of water available for all uses to many of the world’s poor people living in arid regions.

Other water-saving appliances are widely available. Low-flow showerheads can save large amounts of water by cutting the flow of a shower in half. Front-loading clothes washers use 30% less water than top-loading machines use. According to the American Water Works Association, the typical American household could cut its daily water use and water costs by nearly a third by using water-saving appliances and stopping water leaks.

According to UN studies, 30–60% of the water supplied in nearly all of the world’s major cities in less-developed countries is lost, primarily through leakage from water mains, pipes, pumps, and valves. Water experts say that fixing these leaks should be a high priority for water-short countries, because it would increase water supplies and cost much less than building dams or importing water.

Even in advanced industrialized countries such as the United States, losses to leakage average about 16%. However, leakage losses have been reduced to about 3% in Copenhagen, Denmark, and to less than 3% in Fukuoka, Japan. In one year, a faucet leaking water at the rate of 1 drop per second can waste 10,000 liters (2,650 gallons). Not detecting and fixing water leaks from faucets, pipes, and toilets is equivalent to burning money.

13.5e

Using Less Water to Remove Wastes

Currently, we use large amounts of freshwater to flush away industrial, animal, and household wastes. According to the UN Food and Agriculture Organization (FAO), if current growth trends in population and water use continue, within 40 years, the equivalent of the world’s entire reliable flow of river water will be needed just to dilute and transport such wastes.

Recycling and reusing gray water from sewage plants could save much of this freshwater. In Singapore, all sewage water is treated at reclamation plants for reuse by industry. U.S. cities such as Las Vegas, Nevada, and Los Angeles, California, are also beginning to clean up and reuse some of their wastewater. However, less than 10% of the water in the United States is recycled, cleaned up, and reused. Sharply raising this percentage would be a way to apply the chemical cycling principle of sustainability.

Another way to keep freshwater out of the waste stream is to rely more on waterless composting toilets. These devices convert human fecal matter to a small amount of dry and odorless soil-like humus material that can be removed from a composting chamber and returned to the soil as fertilizer. One of the authors (Miller) used a composting toilet for over a decade with no problems, while living and working deep in the woods in a small passive solar home and office used for evaluating solutions to water, energy, and other environmental problems (see p. xxxiii).

As water shortages grow in many parts of the world, people are using methods discussed here to use water more sustainably. Their experiences can be instructive to people who want to avoid water shortages in the first place.

Learning from Nature

The leaves of the Lotus flower do not absorb water. Tiny bumps on a lotus leaf, coated with a natural waxy material, cause water to roll off, taking any dirt particles with it, which makes the leaf self-cleaning. Scientists have mimicked this effect to create a water-, fat-, and oil-repellant sealant that can be sprayed onto a surface. This could allow for making self-cleaning products that would require little or no water for cleaning. 13.6a

Some Areas Get Too Much Water

Some areas have too little freshwater. Other areas sometimes have too much because of natural flooding by streams, caused mostly by heavy rain or rapidly melting snow. A flood happens when freshwater in a stream overflows its normal channel and spills into an adjacent area, called a floodplain.

Natural flooding, which occurs on many streambeds every spring, provides several benefits. It has created some of the world’s most productive farmland by depositing nutrient-rich silt on floodplains. It also helps recharge groundwater and refill wetlands that are commonly found on floodplains, thereby supporting biodiversity and aquatic ecosystem services.

People settle on floodplains to take advantage of their many assets such as fertile soil suitable for crops, but certain human activities have led to increased flooding in many areas. Major floods kill thousands of people every year and cost tens of billions of dollars in property damage (see the Case Study that follows). Such floods are usually considered natural disasters, but since the 1960s, human activities have contributed to a sharp rise in flood deaths and damages, meaning that such disasters are partly caused by human actions.

Case Study

Living Dangerously on Floodplains in Bangladesh

Bangladesh is one of the world’s most densely populated countries. In 2018, its 166 million people were packed into an area roughly the size of the U.S. state of Wisconsin (which has a population of less than 6 million). In addition, the country’s population is projected to increase to 202 million by 2050. Bangladesh is a flat country, only slightly above sea level, and it is one of the world’s poorest countries.

The people of Bangladesh depend on moderate annual flooding during the summer monsoon season to grow rice and help maintain soil fertility in their country’s delta basin region, which is fed by numerous river systems. The annual floods also deposit eroded Himalayan soil on the country’s crop fields. Bangladeshis have adapted to moderate flooding. Most of the houses have flat thatch roofs on which families can take refuge with their belongings in case of rising waters. The roofs can be detached from the walls, if necessary, and floated like rafts. After the waters have subsided, the roof can be reattached to the walls of the house. However, great floods can overwhelm such defenses.

In the past, great floods occurred every 50 years or so. However, between 1987 and 2017 there were eight severe floods, each covering a third or more of the country with water. Bangladesh’s flooding problems begin in the Himalayan watershed, where rapid population growth and unsustainable farming have resulted in deforestation. Monsoon rains now run more quickly off the barren Himalayan foothills, carrying vital topsoil with them (Figure 13.27, right).

Figure 13.27

Natural capital degradation: A hillside before and after deforestation.

Question:

How might a drought in this area make these effects even worse?

An illustration shows two models, one for forested hill-side and the other for After Deforestation. An illustration for forested hillside shows a lake, crops cultivated in agricultural land, and a grassland at the base of the hill and along the elevation of the hill-side, numerous trees are shown. Cloud mass is shown above the hill from which raining is shown. Arrow marks pointing upwards from the hillside is shown and the text beside reads, “Evapotranspiration.” The agricultural land is labeled. The text over the hill-side reads, “Diverse Ecological Habitat.” A callout pointing to the tree roots reads the text, “Tree roots stabilize soil.” A callout pointing to the grasslands reads the text, “Vegetation releases water slowly and reduces flooding.” The text beside the model reads, “Trees reduce soil erosion from heavy rain and wind.” An illustration on the right side, shows After Deforestation displaying the hillside which is covered with trees as well as roads and at the base of the hillside are the lake, grasslands, and agricultural land. Along the slope of the hillside, gullies and landslides are shown and labeled. The grasslands are shown as bare lands and labeled as, “Heavy rain erodes top soil.” The lake is labeled as, “Silt from erosion fills rivers and reservoirs.” The region close to the lake-side is labeled as, “rapid runoff causes flooding.” The text on the top of the agricultural land reads, “Agricultural land is flooded and silted up.” The text beside the grassland where the cattle are grazing reads “Overgrazing accelerates soil erosion by water and wind.” Below that it is labeled as, “winds remove fragile topsoil.” The sand path on the sides of the grassland is labeled as, “Gullies and landslides.”Enlarge Image

This increased runoff of topsoil, combined with heavier-than-normal monsoon rains, has led to more severe flooding along Himalayan rivers, as well as downstream in Bangladesh’s delta areas. In 1998, a disastrous flood covered two-thirds of Bangladesh’s land area, in some places for 2 months, drowning at least 2,000 people and leaving 30 million homeless. It also destroyed more than one-fourth of the country’s crops, which caused thousands of people to die of starvation. Another flood in 2017 affected at least 8 million people by leaving hundreds of thousands homeless and destroying crops and access to clean water.

Many of the coastal mangrove forests in Bangladesh (and elsewhere; see Figure 8.8) have been cleared for fuelwood, farming, and shrimp farming ponds. The result: more severe flooding because these coastal wetlands had helped to shelter Bangladesh’s low-lying coastal areas from storm surges, cyclones, and tsunamis. In areas of Bangladesh still protected by mangrove forests, damages and death tolls from cyclones have been much lower than they were in areas where the forests have been cleared.

Projected rises in sea level and storm intensity during this century, primarily due to projected climate change, will likely be a major threat to Bangladeshis who live on the flat delta adjacent to the Bay of Bengal. This would create millions of environmental refugees with no place to go in this already densely populated country.

Bangladesh is one of the few less-developed nations that is implementing plans to adapt to projected rising sea levels. This includes using varieties of rice and other crops that can better tolerate flooding, saltwater, and drought. People are also building ponds to collect monsoon rainwater and a network of earthen embankments to help protect against high tides and storm surges. Bangladesh has been praised in recent years for its work on disaster preparedness, including construction of storm shelters and improved evacuation procedures. Such measures have resulted in declining death tolls and property damage despite more frequent storms and flooding.

Human activities contribute to flooding in several ways. First, in efforts to reduce the threat of flooding on floodplains, some rivers have been narrowed and straightened, banked by protective dikes and levees (long mounds of earth along their banks), and dammed to create reservoirs that store and release water as needed. However, such measures can lead to greatly increased flood damage when heavy snowmelt or prolonged rains overwhelm dikes and levees.

Second, some human activities include the removal of water-absorbing vegetation, especially on hillsides (Figure 13.27). Once the trees on a hillside have been cut for timber, fuelwood, livestock grazing, or farming, freshwater from precipitation rushes down the naked slopes, eroding precious topsoil. This practice can increase flooding and pollution in local streams. Such deforestation can also make landslides and mudflows more likely.

A third human activity that increases the severity of flooding is the draining of wetlands that naturally absorb floodwaters. These areas often end up covered with pavement and buildings. This leads to greatly increased runoff, which contributes to flooding and pollution of surface water. When Hurricane Katrina struck the Gulf Coast of the United States in August 2005 and flooded the city of New Orleans, Louisiana, the damage was intensified because of the degradation or removal of coastal wetlands. These wetlands had historically helped to absorb water and buffer this low-lying land from storm surges. For this reason, Louisiana officials are now working to restore some coastal wetlands.

A fourth human-related factor likely to increase flooding is a rise in sea levels, projected to occur during this century mostly because of climate change related to human activities. Climate change models project that, by 2075, as many as 150 million people living in the world’s largest coastal cities could be flooded out by rising sea levels.

According to many scientists, we can reduce flooding and water pollution by relying less on engineered devices such as dams and levees and more on nature’s systems. By preserving existing wetlands and restoring degraded wetlands that lie in floodplains, we can take advantage of the natural flood control they provide. These and other ways to reduce our contribution to flooding are listed in Figure 13.28.

Figure 13.28

Methods for preventing or reducing the harmful effects of flooding.

Critical Thinking:

Which two of these solutions do you think are the best ones? Why?

An illustration shows information about Reducing Flood Damage in three columns. The text in the first column provides information about Prevention which reads, “Preserve forests in watersheds, preserve and restore wetlands on floodplains, tax development on floodplains, increase use of flood plains for sustainable agriculture and forestry.” The second column shows the photo of a river, flowing along with green vegetation on the sides, and another photo which shows a dam site comprising of the river and dam construction. The third column provides information about Control that reads the text, “Strengthen and deepen streams (channelization), build levees or floodwalls along the streams, and build dams.”

Top: allensima/ Shutterstock.com. Bottom: Zeljko Radojko/ Shutterstock.com.

13.6b

Reducing Flood Risks

Many scientists argue that we could improve flood control by relying less on engineered devices such as dams and levees and more on nature’s systems such as wetlands and forests in watersheds.

One engineering approach is the channelizing of streams, which does reduce upstream flooding. However, it also eliminates the aquatic habitats that lie along a meandering stream by taking the water from those systems and sending it in a faster flow straight down a channel. It also reduces groundwater recharge and often leads to downstream flooding.

Similarly, levees or floodwalls along the banks of a river contain and speed up stream flow and can lead to flooding downstream. They also do not protect against unusually high and powerful floodwaters such as those that occurred in 1993 when water flowed over two-thirds of the levees along the Mississippi River.

Damming, the most common engineering approach, can reduce the threat of flooding by storing water in a reservoir and releasing it gradually. However, dams also have a number of drawbacks (Figure 13.15).

An ecologically oriented approach to reducing flooding is to preserve existing wetlands and restore degraded wetlands that lie in floodplains to take advantage of the natural flood control they provide. We would also be wise to sharply reduce emissions of greenhouse gases that contribute to atmospheric warming and climate change, which will likely raise sea levels and flood many of the world’s coastal areas during this century.

Figure 13.28 summarizes these various ways to reduce flooding risks.

Big Ideas

One of the major global environmental problems is the growing shortage of freshwater in many parts of the world.

We can expand water supplies in water-short areas in a number of ways, but the most important ways are to reduce overall water use and to use water much more efficiently.

We can use water more sustainably by reducing water use, using water more efficiently, cutting water losses, raising water prices, and protecting aquifers, forests, and other ecosystems that store and release water. This chapter’s Core Case Study discusses the problems and tensions that can occur when a large number of U.S. states share a limited river water resource in a water-short region. Such problems are representative of those faced by many other dry regions of the world, especially areas where the population is growing rapidly and water resources are dwindling for various reasons.

Large dams, river diversions, levees, and other big engineering schemes have helped to provide much of the world with electricity, food from irrigated crops, drinking water, and flood control. However, they have also degraded the aquatic natural capital necessary for long-term economic and ecological sustainability by seriously disrupting rivers, streams, wetlands, aquifers, and other aquatic systems.

The three scientific principles of sustainability can guide us in using water more sustainably during this century. Scientists hope to use solar energy to desalinate water and expand freshwater supplies. Recycling more water and reducing water waste will help reduce water losses. Preserving biodiversity by avoiding disruption of aquatic systems and their bordering terrestrial systems is a key factor in maintaining water supplies and water quality. Chapter Review

Doing Environmental Science

1. Investigate water use at your school. Try to determine all specific sources of any water losses, taking careful notes and measurements for each of them, and estimate how much water is lost per hour, per day, and per year from each source. Sum these estimated amounts to arrive at an estimate of total water losses for a year at your school. Develop a water conservation plan for your school and submit it to school officials.

Chapter Review

Ecological Footprint Analysis

The following table is based on data from the Water Footprint Network, a science-based organization that promotes the sustainable use of water through sharing knowledge and building awareness of how water is used. It shows the amounts of virtual water (also called embedded water), or water required to produce each of the products listed in the first column. Study the table and then answer the questions that follow.

Product

Liters per kilogram (kg) or product

Gallons per pound (lb) or product

Beef

15,400/kg

1,855/lb

Pork

5,990/kg

722/lb

Chicken

4,325/kg

521/lb

Milk

255/glass (250 ml)

68/glass (8 oz)

Eggs

196/egg

52/egg

Coffee

132/cup

35/cup

Beer

74/glass (250 ml)

20/glass (8 oz)

Wine

54/glass (250 ml)

14/glass (8 oz)

Apple

125/average size apple

33/apple

Banana

160/large banana

42/banana

Tomato

50/medium tomato

13/tomato

Rice

2,500/kg

301/lb

Bread

1,608/kg

426/lb

Cotton t-shirt

2,495/shirt

661/shirt