Environmental Science: Water Use

Consumptive Use of Renewable Water Supplies

A region's renewable water supply is all the water that enters the region by precipitation and other imports minus the water that leaves the region by evaporation, plant use, and other exports. Water is a renewable resource that, if used with care, will always be available to us. However, as seen on this map, consumptive water use can exceed the renewability of a region's water supply, which disrupts the balance between the water coming in and the water being used. Notice for example that in the Lower Colorado River basin, daily water consumptive use is 10.6 billion gallons, whereas the region's renewable water supply is only 10.3 billion gallons per day, which causes a 300-million-gallon deficit.

Agricultural Water Use

Because of the strategies that most farmers use to water and take care of their crops, agriculture accounts for most of the consumptive water use on the Earth. Two of the most common ways to water crops is to simply flood the whole field or run water in rows between crops, which are both highly inefficient because approximately half of the water evaporates and is not returned to the original source. A percentage of the remaining water runs off the field before it is used, and another large portion of it becomes contaminated with pesticides and fertilizers. The need for food production increases with the ever-increasing worldwide population growth. This increases the demand to pump water from aquifers. The Ogallala Aquifer is an example of an aquifer that is being used up faster than it is being replaced by rain and snow. The Ogallala, an important water resource for agriculture and ranching in the mid-western United States, is at risk for drying up within the next 25-30 years according to some geologists.

What You Should Know

Before you begin this lesson, you should: have a firm understanding of the different sources of water on Earth.

Think About It

Can you think of ways in which farmers can reduce the amount of consumptive water used for agricultural production? We have to produce food, but as you learned in the previous lesson, worldwide supplies of water are often limited and increased pumping from aquifers can lead to their depletion. How can we make better use of agricultural water? Brainstorm some ideas and post your ideas in your notes. If you need some inspiration, do a web search on "water conservation—agriculture."

Domestic Water Use

Domestic water use is the quantity of water used by people in cities and towns and in their private homes. People in developed countries use about ten times more water than those in developing countries. This difference is due to the income level of the people in an area and the amount of available water. Highly affluent or wealthy countries use much more water than poor countries because they can afford to waste it. Also, countries that simply have more water to start with use more than those countries that have scarce water resources. For example, the average American uses about 1800 gallons of water every day, while in Ethiopia, water use is as low as 1 gallon per day per person. These trends are related to income level, initial water supply, and climate.

OBJECTIVES

Explain the global uses of water. Identify the largest consumers of water. Describe how factors such as climate, income level, and degree of industrialization relate to water use for a country. Distinguish between consumptive and non-consumptive water use. Describe the consequences of depleting groundwater resources.

Household Water Use

Finally, it's time to tie up the use of global and American use of water with your individual experience. This graph depicts how a typical American household uses water. Think about the amount of water you use in a single day. Imagine a large urban area, with a population of several million people who use water just the way you do. Now imagine how much water could be saved if everyone in the area took shorter showers or fixed leaky faucets. In the next lesson, you will learn ways to conserve, or save, water.

LET'S REVIEW!

In this lesson, you have covered the following points: Agriculture is the largest use of water worldwide. A large percentage of water used by agriculture is consumptive water, meaning that the water does not get returned to its source. Non-consumptive water use involves the return of water to its source after use. Water consumption can sometimes exceed water renewability, leading to water deficits and hardships for people and economies. Some consequences of over pumping groundwater include subsidence, sinkholes, and salt water intrusion. FAQs Is the Aral Sea still shrinking? Yes, the southern part of the Aral Sea is shrinking even faster than once predicted, and it has many catastrophic effects. How does subsidence affect an area? Land subsidence can have serious consequences for homes and other buildings. Imagine what would happen to your home if the land under it starting sinking. Subsidence causes millions of dollars of damage to building structures worldwide.

Depleting Groundwater

Nearly 40 percent of the U.S. population depends on groundwater for freshwater use. In rural areas, this number is as high as 95 percent, but humans have continued to deplete these sources also. Look at each image to learn more about problems associated with rapid groundwater depletion.

GLOBAL WATER WITHDRAWAL

Now that you have learned about the water withdrawal of the United States, it is time to draw your attention to the state of the global community. This diagram shows how water withdrawal compares worldwide. For each continent, the darker shaded bar represents withdrawal, and the lighter shaded bar represents consumptive use. Study the diagram and answer the self-check questions.

Saltwater Intrusion

Saltwater intrusion occurs when freshwater is overdrawn along coastal areas. Saltwater from nearby oceans or gulfs moves in to take the place of the missing freshwater. Because saltwater cannot be used for drinking, saltwater intrusion results in a diminished supply of fresh drinking water that can lead to water shortages and illness.

Water Consumption

So now that you know that agriculture consumes 70 percent of water in the world. The industrial use of water consumes approximately 23 percent, and the domestic sector uses the remaining 7 percent of water. So where does the water come from? Agriculture, industry, and cities take water from sources like aquifers; this is called water withdrawal. Water consumption means how much of the water withdrawn that is not returned to its original source, and the water that does not return is called consumptive water. Consumptive water is lost by evaporation, transported to another area, or contaminated, which disrupts the water cycle. A large portion of the total water withdrawn for agriculture is consumptive water, and a lower percentage of water withdrawn for domestic and industrial uses is also consumptive water. The opposite of consumptive water is non-consumptive water. Water use is non-consumptive if it results in the water being returned to its source after use. For example, your household water may come from a local river. After you flush the toilet, the water will go to a waste water treatment plant, be cleaned up, and returned to the river. Because this use eventually results in the water going back to its source, it is non-consumptive.

Water Usage

The graph shows you how water is used by three main sectors: agriculture, industry, and residential use. It also provides information on how much water is lost from our global reservoirs by evaporation. Agriculture use includes any use associated with watering cropland, giving water to livestock animals, cleaning up the wastes of animals, and transporting and producing agricultural products. Industry use includes any use associated with the production and transportation of materials. Domestic use includes things like drinking water, water for bathing, and water for lawns and car washing in our private households. It also includes use by cities.

Aral Sea Case Study

The limited supply of freshwater on Earth can cause catastrophic problems because of its importance to all life. One of these problems occurs when the depletion of groundwater is faster than precipitation can cycle water back into the aquifer. Depletion of groundwater and other freshwater and saltwater sources increases when competing human populations share a limited source of water. The Aral Sea, located in southwest Asia, was once the world's fourth largest inland saltwater sea. Since 1960, it has lost more than 75 percent of its water. This is because the Amu-Daria and Syr-Daria rivers that flow into it have been continuously pumped to provide water for crop irrigation. Water that once fed the Aral Sea is now being used to grow cotton and rice crops along these rivers, which has caused many unforeseen problems for both people and the environment. In the past, the people who lived near the Aral Sea were able to depend on a profitable fishing economy, but now they have almost nothing left to fish. This has caused extensive economic problems, including an increase in poverty levels. Many villagers had to leave their towns completely because of lack of work and food. Others have had to adapt to the increasingly arid nature of the land. The environment has also been affected by the shrinking of the Aral Sea. The white areas that you see on the 2008 image represent soil that is covered with salt that has been left behind by the shrinking sea. Now this soil is too salty for growing crops, and many people are unable to farm. In addition to an excess of salts, the fertilizers and pesticides used for irrigation in the past have polluted the drinking water. Many villagers have no other source for water and therefore are forced to drink from this tainted water source.

Sinkholes

This picture shows a sinkhole, which forms when the roof of an underground channel or cavern collapses. Several things can cause sinkholes, and they do occur naturally without human influence. However, over pumping groundwater can be a contributing factor to sinkhole formation, especially in large urban areas where there is a large demand for groundwater. If one forms where homes or businesses are located, it can be a serious natural disaster, resulting in death and the mass destruction of property.

The Blue Earth

Water makes up 71 percent of the surface of Earth, and humans and all other forms of life are completely dependent on water. Which of the following do you think is the number one way humans use water on a worldwide basis? 1) agriculture (growing and producing food) 2) industry (making materials and products) 3) residential (for human drinking and household use) Answer: 1. That's right. Worldwide agriculture accounts for almost 70 percent of all water use.

Land Subsidence

When aquifers are over pumped, the surface above them begins to settle or sink. The U.S. Geological Survey estimates that the San Joaquin Valley in California has sunk more than ten meters in the last fifty years because of excessive groundwater pumping. The picture gives you a startling view of just how much the land in that area has sunk since 1925. This sinking of the land is called a subsidence, which can have devastating effects.

WHERE DOES WATER GO?

Which of these two objects do you think requires the most water for its production? In the "Prepare" section of this lesson, you learned that agricultural food production is the number one consumer of water worldwide. This rib-eye steak may have required up to 33,000 gallons of water for its production; while on the other hand, an equal weight of soft drink cans require only about 2,800 gallons of water to produce. Agriculture consumes approximately 70 percent of the Earth's water to irrigate cropland and produce food and other important crops. An example of the high water cost of agriculture is that it can take 1,000 metric tons of water to produce just one metric ton of grain.

Industrial Water Use

Worldwide, industry accounts for about 24 percent of all water use, but this is an average amount. Industrial water use can be as much as 70 percent in some highly industrialized European countries or as low as 5 percent in less industrialized countries, such as Bolivia. Most of industrial water is used as a cooling agent for power plants, but unlike agricultural water use, the majority of the water is non-consumptive. Industrial plants usually draw water out of a river or ocean, use it for cooling their equipment, and then return the water to its origins. If care is taken to avoid contamination, the water is not degraded or polluted, and it can be used again for other purposes.

developing country

a country that is poor and whose citizens are mostly agricultural workers, but that is becoming more advanced socially and economically

sinkhole

a large hole that opens up in the ground, usually caused by the collapse of the roof of a cavern or underground cave

renewable water

all the water that enters an area by precipitation and other imports minus the water that leaves the area by evaporation, plant use, and other exports

developed country

another name for a high-income country, that is, a country with a relatively high level of per capital income

domestic

related to private households and cities

saltwater intrusion

the movement of saltwater into places that were once occupied only by freshwater

industry

the people or companies engaged in a particular kind of commercial enterprise

agriculture

the science, art, and business of cultivating soil, producing crops, and raising livestock

subsidence

the sinking of the land above an aquifer, caused by over pumping groundwater from the aquifer

consumptive water use

water use that does not return water to its source, but instead leads to water being lost to evaporation, transported to another area, or contamination

non-consumptive water use

water use that returns water to its source