(4) Water

Water Supply Video

Water4

 How Can We Increase Water Supplies?

CONCEPT 11-2A Groundwater used to supply cities and grow food is being pumped from aquifers in some areas faster than it is renewed by precipitation.

 CONCEPT 11-2B Using dams, reservoirs, and transport systems to transfer water to arid  regions has increased water supplies in those areas, but has disrupted ecosystems and displaced people.

 CONCEPT 11-2C We can convert salty ocean water to freshwater, but the cost is high, and the resulting salty brine must be disposed of without harming aquatic or terrestrial ecosystems.

  There Are Several Ways to Increase Freshwater Supplies

 The most common ways to increase the supply of freshwater in a particular area are withdrawing groundwater, building dams and reservoirs to store runoff in rivers for release as needed, transporting surface water from one area to another, and converting saltwater to freshwater (desalination) (Concepts 11-2A, 11-2B, and 11-2C). Other important strategies discussed later in this chapter involve water conservation and better use of the natural hydrologic cycle.

  We Are Withdrawing Groundwater Faster Than It Is Replenished in Some Areas

Most aquifers are renewable resources unless their water becomes contaminated or is removed faster than it is replenished by rainfall, as is occurring in many parts of the world. Aquifers provide drinking water for nearly half of the world’s people. In the United States, aquifers supply almost all of the drinking water in rural areas, one-fifth of that in urban areas, and 37% of irrigation water. Relying more on groundwater has advantages and disadvantages. Water tables are falling in many areas of the world because the rate of pumping water (mostly to irrigate crops) from aquifers exceeds the rate of natural recharge from rainfall and snowmelt (Concept 11-2A). The world’s three largest grain producers-India, China, and the United States-and several other countries such as Saudi Arabia, Mexico, and Pakistan are over pumping many of their aquifers. Currently, more than half a billion people are being fed by grain produced through the unsustainable use of groundwater, and this number is expected to grow. Worldwide, the unsustainable depletion of aquifers amounts to water that every day would fill a convoy of large tanker trucks stretching 480,000 kilometers (300,000 miles)-more than the distance to the moon. In the United States, groundwater is being withdrawn on average four times faster than it is replenished (Concept 11-2A). One of the most serious overdrafts is in the lower half of the Ogallala, the world’s largest known aquifer, which lies under eight Midwestern states from southern South Dakota to Texas. Although it is gigantic, the Ogallala is essentially a one-time deposit of liquid natural capital with a very slow rate of recharge. In some areas, farmers are withdrawing water from this aquifer as much as 40 times faster than nature replaces it.

Such over drafting has lowered the water table more than 30 meters (100 feet) in some places. Unless water saving irrigation systems are used, most hydrologists predict that groundwater levels in much of this aquifer will drop to the point where using deep wells to pump the water out will cost more than the water is worth for growing crops and raising cattle. Serious groundwater depletion is also taking place in California’s Central Valley, which supplies half of the country’s fruit and vegetables.

 Groundwater overdrafts near coastal areas can contaminate groundwater supplies by causing intrusion of saltwater into freshwater aquifers, which makes such water undrinkable and unusable for irrigation. This problem is especially serious in the U.S. coastal areas of Florida, California, South Carolina, Georgia, New Jersey, and Texas, as well as in coastal areas of Turkey, Manila in the Philippines, and Bangkok in Thailand. Rising sea levels from global warming will increase saltwater intrusion and can decrease the amount of groundwater available in heavily populated coastal areas.

 If we don’t sharply slow the depletion of groundwater, an increasing number of the world’s people will have to live on rainwater and suffer from decreased food production. Figure 11-10 lists ways to prevent or slow groundwater depletion by using this potentially renewable resource more sustainably. With global water shortages looming, scientists are evaluating deep aquifers-found at depths of 0.8 kilometer (0.5 mile) or more-as future water sources.

 Preliminary results suggest that some of these aquifers hold enough water to support billions of people for centuries. The quality of water in these aquifers may also be much higher than the quality of the water in most rivers and lakes.

Assuming that the costs are not too high, there are two major concerns about tapping these nonrenewable deposits of water. First, little is known about the geological and ecological impacts of pumping water from deep aquifers. Second, some deep aquifers flow beneath several different countries, and there are no international water treaties that govern rights to such water. Without such treaties, conflicts could ensue over who has the right to tap into these valuable resources.

  Large Dams and Reservoirs Have Advantages and Disadvantages

Large dams and reservoirs have both benefits and drawbacks. Their main purposes are to capture and store runoff and release it as needed to control floods, generate electricity, and supply water for irrigation and for towns and cities. Reservoirs also provide recreational activities such as swimming, fishing, and boating. Over the past 50 years, an average of two large dams at least 15 meters (49 feet) high have been constructed somewhere on the earth every day. As a result, reservoirs now hold 3 to 6 times more water than flows in the world’s natural rivers, many of which no longer reach the sea.

More than 45,000 large dams in the world (22,000 of them in China) have increased the annual reliable runoff available for human use by nearly one-third.

But a series of dams on a river and withdrawals of river water for agricultural and urban uses, especially in arid areas, can reduce downstream flow to a trickle and prevent it from reaching the sea as a part of the hydrologic cycle. According to the World Commission on Water in the 21st century, more than half of the world’s major rivers go dry part of the year because of such flow reduction especially during drought years.

 THINKING ABOUT

  Dams and Egypt

 Upstream dams and diversions of water from the Nile River by Ethiopia and Sudan will reduce the water available to Egypt, which cannot exist without such water. Which one or more of the options discussed in the Core Case Study do you think Egypt should pursue? Explain.

Worldwide, this engineering approach to river management has displaced 40-80 million people from their homes, flooded an area of mostly productive land roughly equal to the area of the U.S. state of California, and often impairs some of the important ecological services rivers provide (Concept 11-2B). According to water-resource expert Peter H. Gleck, at least a fourth of the world’s freshwater fish species are threatened or endangered, primarily because dams and water withdrawals have destroyed many free-flowing rivers.

Since 1960, the Colorado River, the largest river in the U.S. southwest, has rarely made it to the Gulf of California because of a combination of multiple dams, large-scale water withdrawal, and prolonged drought. Such withdrawals threaten the survival of species that spawn in the river, destroy estuaries that serve as breeding grounds for numerous aquatic species, and increase saltwater contamination of aquifers near the coast Climate change will heighten shortages of water in many parts of the world. Hundreds of millions of people in China, India, and other parts of Asia depend on river flows fed by melting glaciers in the Himalayas. Many of these glaciers in Asia and in parts of South America are receding and are projected to disappear during this century as the earth’s atmosphere continues to warm.

Global warming also changes the timing of water flows into rivers from melting snow in mountainous areas. As temperatures rise, the melting snow will fill rivers earlier in spring and sharply reduce the supply needed to grow crops during the summer. Alterations in the patterns of precipitation and evaporation caused by climate change from global warming are still poorly understood, but there is general agreement that some areas will get drier and some will get wetter and that this will change where we can grow food. California Transfers Massive Amounts of Water from Water-Rich Areas to Water-Poor Areas Tunnels, aqueducts, and underground pipes can transfer stream runoff collected by dams and reservoirs from water-rich areas to water-poor areas, but they also create environmental problems (Concept 11-2B).

One of the world’s largest water transfer projects is the California Water Project (Figure 11-13). It uses a maze of giant dams, pumps, and aqueducts to transport water from water-rich northern California to water-poor southern California’s heavily populated, arid agricultural regions and cities. This project supplies massive amounts of water to areas that, without such water transfers, would be mostly desert.

For decades, northern and southern Californians have feuded over how the state’s water should be allocated under this project. Southern Californians want more water from the north to grow more crops and to support Los Angeles, San Diego, and other growing urban areas. Agriculture consumes three-fourths of the water withdrawn in California, much of it used inefficiently for water-thirsty crops such as rice and alfalfa growing in desert-like conditions.

Northern Californians counter that sending more water south degrades the Sacramento River, threatens fisheries, and reduces the flushing action that helps clean San Francisco Bay of pollutants. They also argue that much of the water sent south is wasted. They point to studies showing that making irrigation just 10% more efficient would provide enough water for domestic and industrial uses in southern California. According to a 2002 study by a group of scientists and engineers, projected global warming will sharply reduce water availability in California (especially southern California) and other water-short states in the western United States even in the best-case scenario. Some analysts project that sometime during this century, many people living in arid southern California cities such as Los Angeles and San Diego, as well as farmers in this area, may have to move elsewhere for water.

Pumping more groundwater is not the answer because groundwater is already being withdrawn faster than it is replenished in much of central and southern California. It would be quicker and cheaper to improve irrigation efficiency, stop growing water-thirsty crops in arid areas, and increase the historically low price of water to reduce water waste.

CASE STUDY

 The Aral Sea Disaster

The shrinking of the Aral Sea is the result of a large-scale water transfer project in an area of the former Soviet Union with the driest climate in central Asia. Since 1960, enormous amounts of irrigation water have been diverted from the inland Aral Sea and its two feeder rivers to create one of the worlds largest irrigated areas, mostly for raising cotton and rice. The irrigation canal, the world’s longest, stretches more than 1,300 kilometers (800 miles).

This large-scale water diversion project, coupled with droughts 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 tripled and the average level of its water has dropped by 22 meters (72 feet). It has lost 90% of its volume of water and has split into two parts. Water withdrawal for agriculture has reduced the two rivers feeding the sea to mere trickles.

About 85% of the area’s wetlands have been eliminated and roughly half the local bird and mammal species have disappeared. In addition, a huge area of former lake bottom has been converted to a human-made desert covered with glistening white salt. The sea’s increased salt concentration-three times saltier than ocean water-caused the presumed extinction of 20 of the area’s 24 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 the middle of a salt desert.

Winds pick up the sand and salty dust and blow it onto fields as far as 300 kilometers (190 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-a prime example of unexpected connections and unintended consequences. Shrinkage of the Aral Sea has also altered the area’s climate. The once-huge sea acted as a thermal buffer that moderated the heat of summer and the extreme cold of winter. Now 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.

To raise yields, farmers have used more herbicides, insecticides, and fertilizers, which have percolated downward and accumulated to dangerous levels in the groundwater-the source of most of the region’s drinking water.

Many of the 45 million people living in the Aral Sea’s watershed have experienced increasing health problems-including anemia, respiratory illnesses, kidney disease, and various cancers-from a combination of toxic dust, salt, and contaminated water.

Since 1999, the United Nations and the World Bank have spent about $600 million to purify drinking water and upgrade irrigation and drainage systems. This has improved irrigation efficiency and flushed some salts from croplands. A new dike should raise the average level of the small Aral by 3 meters (10 feet). Some artificial wetlands and lakes have been constructed to help restore aquatic vegetation, wildlife, and fisheries. The five countries surrounding the lake and its two feeder rivers have worked to improve irrigation efficiency and to partially replace water-thirsty crops with others requiring less irrigation water. As a result, the total annual volume of water in the Aral Sea basin has been stabilized. Nevertheless, experts expect the largest portion of the Aral Sea to continue shrinking.

 Removing Salt from Seawater Is Costly, Kills Marine Organisms, and Produces Briny Wastewater

Desalination involves removing dissolved salts from ocean water or from brackish (slightly salty) water in aquifers or lakes for domestic use. It is another way to increase supplies of freshwater (Concept 11-2C). One method for desalinating water is distillation-heating saltwater until it evaporates, leaving behind salts in solid form, and condenses as freshwater. Another method is reverse osmosis (or microfiltration), which uses high pressure to force saltwater through a membrane filter with pores small enough to remove the salt. In effect, high pressure is used to push freshwater out of saltwater.

Today about 15,000 desalination plants operate in more than 125 countries, especially in the arid nations of the Middle East, North Africa, the Caribbean, and the Mediterranean. They meet less than 0.3% of the world’s demand for freshwater.

There are three major problems with the widespread use of desalination. One is the high cost, because it takes a lot of energy to desalinate water. A second problem is that pumping large volumes of seawater through pipes and using chemicals to sterilize the water and control algae growth kills many marine organisms. A third problem is that desalination produces large quantities of briny wastewater that contain lots of salt and other minerals. Dumping concentrated brine into a nearby ocean increases the salinity of the ocean water, threatening food resources and aquatic life in the vicinity.

 Dumping it on land could contaminate groundwater and surface water. Some research is being carried out on the economic and ecological feasibility of building desalination plants offshore and pumping the freshwater to the shore. This could help dilute the resulting briny seawater.

 Bottom line: Currently, significant desalination is practical only for water-short, wealthy countries and cities that can afford its high cost (Concept 11-2C).

 RESEARCH FRONTIER

 Developing better and more affordable desalination technologies.

 

 

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