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A human can survive without food for 3 weeks or more, but cannot survive without water for more than a few days. Water is also essential for producing food. In fact, 70 percent of the world’s freshwater consumption is used for agriculture. The remaining 30 percent is split between industrial and household uses, the proportion of which varies from country to country. On average, experts estimate that about 20 percent of the world’s freshwater use is for industry and about 10 percent is for household uses. In this module, we will examine the major uses of water by humans in the areas of agriculture, industry, and households. We will then investigate how water ownership and conservation will determine the availability of water in the future.

Learning Objectives

After reading this module you should be able to

As we have seen with so many other resources, the per capita daily use of water varies dramatically among the nations of the world. FIGURE 28.1 shows total daily per capita use of fresh water for a number of countries, which is known as the water footprint of a nation. This water use reflects the total water use by a country for agriculture, industry, and residences divided by the population of that country. This allows us to compare water use among nations. For example, a person living in the United States, Spain, or Canada uses about three times more water than a person living in Kenya or China.

As we have just noted, the largest use of water worldwide is for agriculture. During the last 50 years, as agricultural output has grown along with the human population, the amount of water used for irrigation throughout the world has more than doubled. Indeed, producing a metric ton of grain (1,000 kg, or 2,200 pounds) requires more than 1 million liters of water (264,000 gallons). Together, India, China, the United States, and Pakistan account for more than half the irrigated land in the world. In the United States, approximately one-third of all freshwater use is for irrigation. Raising livestock for meat also requires vast quantities of water. For example, producing 1 kg (2.2 pounds) of beef in the United States requires about 11 times more water than producing 1 kg of wheat. In this section, we will consider some of the technological advances for irrigating crops that make efficient use of water.

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Irrigation

Since agriculture is the greatest consumer of fresh water throughout the world, agriculture poses a large potential for conserving water. One way to conserve water in agriculture is by changing irrigation practices. There are four major techniques for irrigating crops: furrow irrigation, flood irrigation, spray irrigation, and drip irrigation (FIGURE 28.2).

The oldest technique is furrow irrigation (FIGURE 28.2a), which is easy and inexpensive. The farmer digs trenches, or furrows, along the crop rows and fills them with water, which seeps into the ground and provides moisture to plant roots. Furrow irrigation is about 65 percent efficient; 65 percent of the water is accessible to the plants and the other 35 percent either runs off the field or evaporates.

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Flood irrigation (FIGURE 28.2b) involves flooding an entire field with water and letting the water soak in evenly. This technique is generally more disruptive to plant growth than furrow irrigation, but is also slightly more efficient, ranging from 70 to 80 percent efficiency. In spray irrigation (FIGURE 28.2c), water is pumped from a well into an apparatus that contains a series of spray nozzles that spray water across the field, like giant lawn sprinklers. The advantage of spray irrigation is that it is 75 to 95 percent efficient, but it is also more expensive than furrow or flood irrigation and uses a fair amount of energy.

The most efficient method of irrigation, drip irrigation (FIGURE 28.2d), uses a slowly dripping hose that is either laid on the ground or buried beneath the soil. Drip irrigation using buried hoses is over 95 percent efficient. It has the added benefit of reducing weed growth because the surface soil remains dry, which discourages weed germination. Drip irrigation systems are particularly useful in fields containing perennial crops such as orchard trees, where the hoses do not have to be moved each year in order to plow the field.

Efficient irrigation technology benefits the environment by reducing both water consumption and the amount of energy needed to deliver the water. Many new technologies are being developed to more carefully control when plants are irrigated. As with all human activities, the costs and benefits of each irrigation technique need to be weighed to determine the best solution for each situation.

Hydroponic Agriculture

For some crops, hydroponic agriculture is an alternative to traditional irrigation. Hydroponic agriculture is the cultivation of crop plants under greenhouse conditions with their roots immersed in a nutrient-rich solution, but with no soil (FIGURE 28.3). Water not taken up by the plants can be reused, so this method uses up to 95 percent less water than traditional irrigation techniques. Hydroponic operations can produce more crops per hectare than traditional farms. They can also grow crops under ideal conditions, grow the crops during every season of the year, and often grow the crops with little or no use of pesticides. Hydroponic agriculture is growing in popularity, especially for vegetables; hydroponic tomatoes, for example, have won awards for their flavor. Although the cost of hydroponic agriculture is higher, consumers are willing to pay more for hydroponically grown vegetables and a number of businesses have been successful. One company in Georgia now grows hydroponic vegetables in a huge facility that covers 129 ha (318 acres).

While most water is used for agriculture, the remainder is used for industrial purposes and households.

Industrial Water Use

Water is required for many industrial processes, such as generating electricity, cooling machinery, and refining metals and paper. In the United States, approximately one-half of all water used goes toward generating electricity. It is important to distinguish between water that is withdrawn from and then returned to its source and water that is withdrawn and consumed. For example, water that passes through a turbine at a hydroelectric dam is withdrawn from a reservoir to generate electricity, but that water is not consumed. Rather, it passes from the reservoir through the turbine and back to the river flowing away from the dam. That is not to say that this process has no effect on the environment; recall our discussion of the ecological impacts of damming rivers earlier in this chapter.

Some processes that generate electricity do consume water. This means that some of the water used is not returned to the source from which it was removed, but instead enters the atmosphere as water vapor. Thermoelectric power plants, including the many plants that generate heat using coal or nuclear reactors, are large consumers of water. These plants use heat to convert water into steam that is used to turn turbines. The steam needs to be cooled and condensed before it can be returned to its source. In many plants, this cooling is accomplished by using massive cooling towers. If you have ever seen a nuclear power plant, even from a distance, you may have seen the large plumes of water vapor rising up for thousands of meters from the cooling towers. The towers allow much of the steam from the plant to condense and cool into liquid water, but a large fraction of it is lost to the atmosphere. This water vapor represents the water that is consumed by nuclear reactors (FIGURE 28.4).

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Industrial processes such as refining metals and making paper also require large amounts of water. Copper, used extensively for electrical wiring, requires 440 L (116 gallons) of water per kilogram to refine. Aluminum, used in products as diverse as automobiles and aluminum foil for cooking, requires 410 L (108 gallons) per kilogram. Steel, used to manufacture home appliances, cars, buildings, and other products, requires 260 L (68 gallons) per kilogram. When we use paper, we indirectly use water. A kilogram of paper requires 125 L (33 gallons) of water to manufacture. As we will see later in this chapter, there are opportunities to reduce the use of water during these processes.

Household Water Use

According to the U.S. Geological Survey, household use accounts for approximately 10 percent of all water used in the United States. The quantity of water used in households depends on the types of infrastructure available. Households in less developed countries generally do not have the appliances and bathroom fixtures that are common in more developed countries. As a result of such disparities, per capita household water use varies dramatically among nations. For example, on average, an individual in the United States uses 595 L (157 gallons) per day, whereas an average individual in Kenya uses only 41 L (11 gallons) per day. FIGURE 28.5 shows per capita daily household water use for the United States and 10 other countries.

Indoor use of water is quite similar across the United States since households across the country are typically equipped with bathrooms, washing machines, and cooking appliances. FIGURE 28.6 shows the fraction of indoor water use that goes to each of these functions. Of all household water that is used indoors, 41 percent is used for flushing toilets, 33 percent for bathing, 21 percent for laundry, and 5 percent for cooking and drinking.

Outdoor water use—for watering lawns, washing cars, and filling swimming pools—varies tremendously across the United States by region. In California, for example, the typical household uses about six times more water outdoors than does the typical Pennsylvania family.

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Although drinking water represents a relatively small percentage of household water use, it is particularly important. If you live in a developed country, you may not have given much thought to the availability and safety of drinking water. However, more than 1 billion people—nearly 15 percent of the world’s population—lack access to clean drinking water. Every year, 1.8 million people die of diarrheal diseases related to contaminated water, and 90 percent of those people are children under 5 years of age. This means that 5,000 people in the world die each day in large part because they do not have access to clean water. As we will see in Chapter 14, developed countries typically have modern sanitation systems, stronger environmental laws, and the technologies to remove harmful contaminants and waterborne pathogens from public water supplies. Poor conditions in many developing countries do not provide safeguards that ensure the availability of clean drinking water.

The future of water availability will depend on many things, including how we resolve issues of water ownership, how we improve water conservation, and—as world population grows—how we develop new water-saving technologies.

Water Ownership

Water is an essential resource, but who actually owns it? This is a rather complex question. In a particular area, such as the Klamath River region described at the beginning of the chapter, it is clear that multiple interest groups can claim a right to use and consume the water. However, having a right to use the water is not the same as owning it. For example, regional and national governments often set priorities for water distribution, but of course they have no control over whether or not a particular year will bring an abundance of rain and snow. In California, the state government promises specific amounts of water for cities, suburbs, farmers, and fish, but these promises can exceed the actual amount of water that is available in many years.

Throughout the world, the issues of water rights and ownership have created many conflicts. Earlier in this chapter we discussed India’s plan to divert water from rivers that flow from the Himalayas into Bangladesh (FIGURE 28.7). Since the water originates in India, does India own the water? Does Bangladesh have any legitimate claim to some of this water that its people have relied on for millennia? In the water-poor Middle East, water rights have long contributed to political tensions. In both the 1967 Arab-Israeli War and the 1980s Iran-Iraq War, disputes over water use added to the conflict. Water experts predict that as populations in these arid regions continue to grow, conflicts over water will increase.

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One solution that has been proposed by economists is to allow all interested parties to openly compete for water and let market forces determine its price. In this way, they argue, water could be owned, but the true value of water would be realized and paid for. In 1981, a free-market system of water distribution was initiated in Chile. While water is public property, Chile allows individuals and corporations to buy and sell the water. The idea behind this approach is that having to buy water will encourage more efficient use. While market forces can be useful in determining the appropriate distribution of water among competing needs, government oversight helps to ensure that the needs of the people and the environment are balanced with the needs of agriculture and industry. As we will see in the Science Applied section that follows this chapter (“Is There a Way to Resolve the California Water Wars?”), competing interests are now attempting to implement this more-balanced approach.

Water Conservation

Ultimately, there is a finite amount of water that we all must share. In some regions, such as much of the northeastern United States, water is abundant. In regions where water is scarce, such as the southwestern United States, water conservation becomes more of a necessity.

In recent years, many countries have begun to find ways to use water more efficiently through technological improvements in water fixtures, faucets, and washing machines. In 1994, new federal standards were issued for toilets and showerheads in the United States. For example, a toilet manufactured before 1994 typically uses 27 L (7 gallons) per flush, but toilets manufactured after January 1994 must use 6 L (1.6 gallons) or less per flush, representing a 78 percent reduction in water use. Australia and some countries in Europe and Asia have moved to dual-flush toilets. First invented by an Australian company in 1980, this type of toilet allows the user to push one button for a normal 6 L flush to remove solid waste, but another button produces a much more efficient 3 L (0.8 gallon) flush to remove liquid waste. These dual-flush toilets are increasingly used in the United States. Consumers also have embraced improved efficiencies in showerheads and washing machines. For example, a 10-minute shower with an older showerhead might use 150 L (40 gallons) of water, but revised federal standards for new reduced-flow showerheads call for a 10-minute shower to use no more than 95 L (25 gallons)—a 37 percent reduction in water use. Washing machines are not subject to the new federal standards, but newer, more efficient front-loading machines are now available to consumers, although these more efficient models cost nearly twice as much to purchase. “Do the Math: Selecting the Best Washing Machine” looks at the costs and benefits of these newer, more energy-efficient washing machines.

In countries where a percentage of the population can be considered wealthy, household water uses can include watering lawns and filling swimming pools, both of which require large amounts of water. In some regions of the United States, homeowners have been encouraged, or even required, to plant vegetation that is appropriate to the local habitat. For example, the city of Las Vegas, Nevada, paid homeowners to remove water-intensive turf grass from their lawns and replace it with more water-efficient native landscaping. Changing the vegetation in this manner can result in a savings of 2,000 L of water per square meter (520 gallons per 10 square feet) of lawn per year (FIGURE 28.8).

One of the best ways to reduce industrial water consumption is by producing more efficient manufacturing equipment. In the United States, businesses and factories have achieved more sustainable water use in the last 15 years, mainly through the introduction of equipment that either uses less water or reuses water. For example, industries that need water for cooling machinery have switched from once-through water systems—systems that bring in water for cooling and then pump the heated water back into the environment—to recirculating water systems.

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Some simple ways to conserve water can be used throughout the world. For example, the impervious surfaces of buildings represent a potential water-collecting surface. A gutter system can collect rainwater and channel it into rain barrels or, for greater capacity, a large underground water tank. As we will see in Chapter 14, some countries are now using wastewater for irrigation after sending it through a sewage treatment process. There is also great interest in developing monitoring technologies that will detect leaks in the pipes that distribute water over large areas.

The world’s growing population and the associated expansion of irrigated agriculture have increased global water withdrawals more than fivefold in the last hundred years. Global water use is expected to continue to grow along with the human population through the early part of this century. However, as FIGURE 28.9 shows, despite the growing population in the United States, water withdrawals have leveled off since they peaked in 1980. This is largely a result of greater efficiency in the use of water for agricultural irrigation, electricity generation, and household appliances. Reductions in water use are projected to continue until at least 2020. Of course, the continued development of more efficient technologies would allow additional reductions in water use.