High School Earth Science/Humans and the Water Supply
- Learn how humans use water.
- Discuss how much water is taken up by each water use.
- Explain the difference between consumptive and non-consumptive water uses.
- Discuss three of the most serious issues humans face today, including shortages of fresh water, lack of safe drinking water, and water pollution.
- Discuss why humans are facing water shortages.
- Discuss how water shortages can lead to disputes and even battles between states and countries bordering on the same water source.
- Explain why one fifth of the human population does not have access to safe drinking water.
- Describe the relationship between disease and exposure to unsafe drinking water.
- What is the origin of California's fresh water supply?
Human Uses of Water
All forms of life need water to survive. As humans, we need water to drink or we need to get it from the foods we eat. We also use water for agriculture, industry, household uses, and recreation. Water is continually cycled and recycled through the environment.
Some ways that we use water consume a lot of water that then is lost to the ecosystem and some ways we use water put less demand on our water supplies. Understanding how water cycles and is replaced is important, especially when we look for ways to use less water.
Currently, agricultural uses the most water. Considering different methods of irrigation and times of day to water crops can improve this situation. Farming, growing crops and raising livestock uses more than two thirds of the water used by humans globally.
When water is used but not recycled, the water use is called consumptive. That water is lost to the ecosystem. When excess water is captured or recycled, it is called non-consumptive. As we move to a more sustainable future, we want to be sure as much of our water use is non-consumptive as possible.
What is the most important thing for all life on Earth? Not gold or diamonds. It is water! From the smallest bacteria to the largest trees, all forms of life on Earth depend on water for survival. As humans, we could not survive for more than a few days without drinking water or getting water from the foods we eat.
In addition to our basic survival need for water to drink, people also use freshwater for agriculture, industry and household needs. Across the world, different communities also use water for many kinds of recreational and environmental activities.
Which human activity uses the most water? Not showers, baths, washing dishes, or other household uses. On average, agriculture uses more than two thirds of the water that humans use across the world. Industry and household uses average 15% each. Recreational use and environmental uses average 1% each. (See Figure 21.1)
Some ways that people use water do not use up the water. When you swim in a lake, you do not use up the water. The water is still in the lake when you climb out. In some cases, water can be recycled for reuse. For example, the water you use to brush your teeth or take a bath can be collected through your household pipes and the sewer system, purified and then redistributed for reuse. These are examples of non-consumptive water use. By recycling water, we ultimately reduce our overall water consumption.
Unlike the previous examples, water sprinklers are called consumptive, because much of the water is lost to the air as evaporation. None of the lost water can be captured and reused.
Agricultural Water Uses
Have you ever watched huge sprinklers watering large fields of crops (Figure 21.2)? If you have, try to imagine how much water it takes to water a field compared to taking a shower or bath. You may be surprised to learn that agriculture uses more than two thirds (69%) of the water humans use, globally.
Two of the most popular irrigation methods are overhead sprinklers and trench irrigation. Trench irrigation systems are just that: trench canals that carry water from a water source to the fields. Farmers often chose these methods because they relatively inexpensive. Unfortunately, they are also wasteful of water. Roughly fifteen to thirty-six percent of the water never reaches the crops, because it evaporates into the air or is lost as runoff. When rain or irrigation water is not absorbed by the soil, often it washes valuable soil away.
Giving up irrigation is not a choice for most farmers. A farmer living in a dry region, such as a desert, needs irrigation, just to grow crops. A farmer living in a wetter place would use irrigation to produce more crops or to grow more profitable crops. In some cases, farmers can choose to grow crops that match the amount of rain that falls in that region naturally.
Instead of giving up irrigation, farmers can use less water by choosing more efficient irrigation methods, such as drip irrigation (Figure 21.3). This irrigation system uses pipes and tubes to deliver small amounts of water directly to the soil at the roots of each plant or tree. It wastes less water than sprinklers and trenches, because almost all of the water goes directly to the soil and plant roots.
You might wonder why any farmer would not switch to efficient irrigation methods, since they would save so much water. There are two reasons. First, drip irrigation and other efficient irrigation methods cost more than trenches and sprinklers. Second, in some countries, such as the United States, the government pays for much of the cost of the water that is used for agriculture. Because, farmers do not have to pay the full price of the water they use, they do not have any financial reason or incentive to use less water.
Aquaculture is the name for the type of farming you might do if you were raising fish, shellfish, algae or aquatic plants (Figure 21.4). This is a farming practice where plants and animals that live in water are raised. As the supplies of fish from lakes, rivers, and the oceans dwindle, people are getting more fish from aquaculture. Raising fish instead of hunting for them is a different way of increasing our food resources. The next time you pass the fish display in the grocery store, look for labels for "farm raised" fish. These fish would have been raised in an aquaculture setting.
Some of the most productive aquaculture farming takes place in wetland areas along coastlines. Rivers and streams carry nutrient-rich water into these wetlands, so fish and other animal life thrives. A good supply of nutrients is important when raising a large community of plants or animals. We need to be careful about the wastes that are added to our coastal waters when we increase plant and animal populations in these areas. Aquaculture can be considered a non-consumptive use of water, as long as we keep our coastal waters in good condition.
Industrial Water Use
Industrial water use accounts for an estimated fifteen percent of worldwide water use. Industries include power plants that use water to cool their equipment, and oil refineries that use water for chemical processes (Figure 21.5). Industry also uses water in many manufacturing processes. Looking at water use in a completely different way, hydroelectric power plants are built along rivers and streams to generate energy. This is a very efficient way to use water that is also non-consumptive.
Starting from when you wake up in the morning, count the ways you use water at home (Figure 21.6). You will need to count the water you drink, water used in cooking, bathing, flushing toilets, and even gardening. You will be surprised to notice how many times a day you use water. Have you ever had to go without water? The United States is a developed country. In developed countries, people use a lot of water each day. People living in less developed countries use far less water than people in the United States. Globally, household or personal water use is estimated to account for fifteen percent of world-wide water use.
Some household water uses are considered non-consumptive, because water is recaptured in sewer systems, treated and returned to surface water supplies for reuse. Watering lawns with sprinklers is an exception. Just like sprinkler irrigation on farms, yard sprinklers are consumptive and use large amounts of water.
We all have many ways to lower the amount of water we use at home. Hardware stores sell water-efficient home products, such as drip irrigation to water lawns and gardens, low flow shower heads and low flow toilets. What other ways can you use less water at home?
Which sports use water? Swimming, fishing, and boating are easy examples to think about (Figure 21.7). Do you think playing golf requires water? Actually it does, because we irrigate the golf course in order to keep it nice and green! The amount of water that most recreational activities use is low: less than one percent of all the water we use.
Most recreational water uses are non-consumptive. That would include swimming, fishing, and boating. We can swim, fish, and boat without reducing the water supply. The same is not true for playing golf, which is the biggest recreational water consumer. Golf courses require large amounts of water. Water used for golf courses is generally consumptive, since most of it is lost to evaporation, soil, and runoff.
Environmental uses include activities to create habitat for wildlife, such as building lakes and fish ladders to help fish spawn (Figure 21.9). Most environmental uses are non-consumptive; they account for even less water use than recreation.
California Water Resources
Nevada and other mountain ranges feeds rivers that crisscross the state. Many of these streams feed into the Sacramento River in the northern part of the Central Valley, and the San Joaquin River in the southern portion (Figure 21.9). Virtually all of these rivers are dammed, some more than once, to supply power and water to the cities and farmland of the state.
Groundwater is also an important source of water in California. In a normal year about 40% of the state's water supply comes from groundwater. In a drought year, the number can rise to 60% or more. The largest groundwater reservoirs are found in the Central Valley where thousands of years of snow melt has fed the aquifers. In many locations, much more groundwater is used each year than is available to recharge the aquifer. Subsidence of the land is common in these regions.
Despite these vast water sources, the states large population and enormous agricultural landscape put a strain on the water supply. Water rights in California are complex and controversial. Although about 75% of the water resources are in the northern one-third of the state, the largest usage, about 80%, is in the southern two-thirds. Besides projects that exist to distribute water within the state, a large source of water is the Colorado River, which California must share with five other states and Mexico. The distribution of water resources in the Western United States will be a topic of much discussion in the coming decades.
- Human water use can be lumped into five categories. The uses are arranged in order of greatest to the least amounts of total water use on Earth:
- Agriculture (sixty-nine percent)
- Industry uses (fifteen percent of global water use)
- Home and Personal use (fifteen percent)
- Recreation uses (less than one percent)
- Environmental use (less than one percent)
- Despite California's abundant water supply from surface streams and groundwater, the state has a number of water rights issues that will be important long into the future.
- Describe the three water uses that consume the most fresh water.
- Explain why humans are limited to using less than one percent of all the water on Earth for our needs.
- List two reasons why human water use has increased tremendously during the past century.
- Describe four consequences of water shortages.
- What does the phrase "water is more valuable than gold" mean?
- Describe why some water uses are called consumptive.
- Describe drip irrigation and why it wastes less water than irrigating with sprinklers.
- Describe why droughts are more serious in arid regions of the world than in wetter regions.
- What is the origin of California's fresh water sources?
- Water use where water is "lost" to evaporation.
- drip irrigation
- Pipes and tubes that deliver small amounts of water directly to the soil at the roots.
- A financial benefit for taking a particular action.
- Water use that does not "use up" the water supply.
Points to Consider
- How could fresh water be more valuable than gold or a diamond?
- Which human activity uses more water than all other activities combined?
- Why don't all farmers use drip irrigation and other water efficient irrigation methods?