(9) Water



Water: the liquid of life

Water is quite possibly the most important resource the environment provides. All living things need water in order to survive. The good news is that the Earth contains a lot of water. The problem is that more than 97 percent of the Earth’s water is salt water in the oceans and, therefore, isn’t drinkable. Living things need fresh water, water without high levels of salt, to live. I explain where freshwater resources are located and how humans access the fresh water they need to drink, bathe, and grow food. I also describe regions of the world that don’t have access to adequate supplies of fresh water and explain how environmental scientists seek to create fresh water from salt water.

Thirsty for More: The Never-Ending Need for Fresh Water

Environmental scientists approach freshwater resources with an understanding that both human societies and surrounding ecosystems need access to a certain amount of water. Thus, people need to share the available fresh water with each other and with all the other organisms that inhabit the environment. I explain how scientists define the use of fresh water in the environment and describe the most common ways in which people use freshwater resources.

Withdrawal versus consumption

Scientists define the use of the Earth’s freshwater resources in two ways:

Withdrawal: Water withdrawal measures the total amount of water removed from its natural source (such as a lake or river). Water that’s withdrawn can be used and returned to its source for reuse.

Consumption: Water consumption measures the amount of water lost (through evaporation, absorption, or chemical transformation) during use. Water that’s consumed can’t be returned to its source and reused. Water that’s withdrawn but not consumed may be degraded or polluted. When this water is returned to its natural source, it’s no longer suitable for human or ecosystem use, but it hasn’t been consumed. In many cases, fresh water is a renewable resource, meaning that it can be recycled and reused repeatedly or that its supplies will be naturally replenished after people (and other organisms) use it. But sometimes the need for water exceeds the availability of local fresh water. This situation - where there isn’t enough water to meet the needs of people and ecosystems - is called water scarcity. Water scarcity can occur for two reasons:

Not enough local water is available to meet the needs of people and ecosystems.

The available water is polluted or otherwise can’t be used to meet every need.

Situations of water scarcity can lead to water stress, which is when inadequate water supply leads to competition and conflict as people try to find ways to meet their water needs. Water stress is most common in regions where the amount of fresh water per person is low, and it can increase even more during years of drought (when seasonal water from rain is absent or lower than expected).

Meeting human water needs

Scientists divide the different ways people use water into three categories: agricultural, domestic (or household), and industrial. I describe these three uses in the following sections.

Watering the crops: Agricultural uses

If you’ve ever owned a houseplant or tried to maintain a green lawn or garden, you know that plants need water. Thus, you may not be surprised to hear that agriculture, the growing of plants as food, is the largest consumer of fresh water on Earth, accounting for nearly 70 percent of all freshwater withdrawal. One of the biggest challenges of farming in some regions of the world is locating enough fresh water to support crops. In the drive to meet the food needs of growing human populations, farmers have extended their croplands into drier regions that are farther from natural, seasonal sources of water. As a result, farmers have to build irrigation systems to bring water to the crops in these drier regions. Irrigation systems come in many different forms, depending on the landscape, the regional water availability, and the water needs of the crops. A few of the most common types of irrigation systems are

Furrow irrigation: Furrow irrigation involves digging furrows, or channels, alongside rows of crops. It’s one of the oldest methods of irrigation and was used by ancient civilizations in Egypt and Mesopotamia. By digging shallow ditches along a gentle slope, farmers rely on the pull of gravity to transport the water from a nearby river or stream into their crop fields. The main problem with furrow irrigation is that it isn’t the most efficient way to water crops. In some regions, as much as 35 percent of the water transported to the crops evaporates or runs off the field without being absorbed into the soil.

Flood irrigation: Flood irrigation uses a natural source of nearby flowing water, such as a river or stream, and periodically diverts the water to flood agricultural fields. This irrigation method allows the water to completely cover and soak into the fields. It’s more efficient than using furrows because it loses only 15 to 20 percent of the water to evaporation or runoff.

Drip irrigation: Drip irrigation applies small amounts of water more directly to the plants that need it. This localized irrigation system uses hoses and pipes to drip water onto (or just below) the soil surface. Losing only 5 percent of the water to evaporation, drip systems are very efficient. They work best in fields that don’t need to be plowed every season because the drip hoses are woven through the field at or below the soil surface.

Sprinklers: Like drip irrigation systems, sprinklers use pipes and hoses to move water. But unlike drip irrigation systems, sprinklers spray water over the fields from above and, thus, require a form of energy to pump the water through the pipes. The efficiency of sprinkler systems varies: Some systems lose up to 25 percent of the water, while others lose only about 5 percent. In large agricultural fields, farmers often use sprinklers that are mounted on wheeled systems that move through the fields. Another common sprinkler is the travelling sprinkler system, which sprays water from a long arm that pivots around a center point. If you’ve ever seen a bird’s-eye view of agricultural fields, you may have noticed fields laid out in circles across the landscape; this circular layout is a result of the travelling sprinkler system, which effectively waters a circle of crops from its center pivot point.

Determining which irrigation system is most sustainable for a particular region depends on many factors, including the availability of water and energy resources, the size and layout of the coverage area, the system costs, and the overall efficiency (which depends, in part, on local soil and weather conditions).

The development of hydroponic agriculture offers a new approach to reducing agricultural water use. A hydroponic system grows crops in a greenhouse without using soil. Instead of soil, the crops are “planted” in nutrient-rich water. The water not used by the plants is recycled and reused, and the growing conditions are controlled from above (by the greenhouse) and below (by the nutrient solution) to be ideal for maximum crop production. Hydroponic agriculture requires extra costs upfront to set up the greenhouse facility, but in the long term, the method saves water and soil resources and also reduces the need for pesticides.

Washing and flushing: Domestic uses

The second largest consumer of fresh water is you (and every other person in the U.S.). Every day you drink water, brush your teeth, wash your clothes, flush the toilet, and bathe. These types of household or domestic water use account for more than 10 percent of the freshwater use in the U.S. The actual use of water in a household depends on what type of plumbing and sewage infrastructure it has. For example, in regions that don’t have indoor plumbing, households don’t use water for flushing toilets. In addition, the graph in Figure 9-1 doesn’t include outdoor household water use, such as watering the lawn or garden, which accounts for about 25 percent of total household use (on average). I explain how to reduce the consumption of household water in the later section “Conserving Fresh Water” and in the sidebar “Do-it-yourself water conservation.”

Keeping things cool: Industrial uses

Various industries use water to produce energy, refine metal, and manufacture products. Most of the industrial water use in the U.S. is for the production of electricity, either through hydropower dams or at power plants. Although the capture of energy from moving water, called hydropower, doesn’t consume water because the water is still available for other uses in the ecosystem, other sources of electricity do consume water. Nuclear reactors and coal power plants, for example, consume fresh water, meaning that the water these industries use for the production of electricity is no longer available for other uses. Both nuclear and coal plants transform water into steam to power engines that generate electricity. In the process, most of the steam is lost to the atmosphere; only some of it is collected, converted back to a liquid, and returned to its source.

Other industrial uses result in water waste or the pollution of water by metals or chemicals. Mining for ores requires a large amount of water to rinse unwanted minerals away from the desired metal resource. Once mined, these metal resources must be refined and manufactured into products, such as aluminium foil, appliances, and cars. This type of industrial water use is a common source of water pollution.

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