The water-energy connection describes how water is used to create energy, such as electricity and fuels like oil and natural gas, and how energy is needed to get water, clean it, move it, heat or cool it, treat it, and dispose of it. This process is sometimes called energy intensity. Energy is used in every step of the water cycle, including getting water, moving it, cleaning it, heating it, and handling wastewater. However, the water used for energy production does not always have to be the same water used for other tasks, even though all energy production requires some water. This makes the connection between water and energy very strong and hard to separate.

One of the first studies to examine this connection was a life-cycle analysis done by Peter Gleick in 1994. This study showed how water and energy depend on each other and started research that looks at both together. In 2014, the US Department of Energy (DOE) released a report on the water-energy connection. The report said that better policies for water and energy, along with more understanding of how this connection might be affected by climate change, are important for national security. A diagram in the DOE’s 2014 report shows how water and energy move through different parts of the US. It highlights that thermoelectric power, which uses water mainly for cooling, is the largest user of water in the country.

Water used in the energy sector

All types of power generation use water for different purposes, such as processing raw materials, building and maintaining power plants, or generating electricity. Renewable energy sources like solar panels and wind turbines need little water to create electricity, but they still use water during the production of materials. Water can be classified into categories such as fresh, ground, surface, blue, grey, or green. Water is considered "used" when it is taken from a source and returned to the same place without reducing the amount available for others, such as in power plants that use water for cooling. These plants are the largest users of water. Although used water is returned to the system, it is often polluted or altered in some way, which affects the environment even if the total amount of water remains the same. Water is "consumed" when it is completely removed from the system, such as through evaporation or when used by plants or people. When evaluating water use, factors like how water is used or consumed, along with how water use changes over time and in different areas, must be considered. This makes it difficult to determine exact water use. According to the International Energy Agency (IEA), water shortages can also affect the movement of fuels and materials. In 2022, droughts and extreme heat caused low water levels in European rivers like the Rhine, which limited the transport of coal, chemicals, and other goods by boat.

Spang et al. (2014) studied how much water is used globally to produce electricity. Their research showed differences in the types of energy produced by countries and how efficiently power is made using water. Managing water and power systems during emergencies with limited water or electricity is important for improving the stability of the connection between water and energy. Khatavkar and Mays (2017a) developed a method to control water and power systems during droughts or electricity shortages to ensure power plants receive enough cooling water. Khatavkar and Mays (2017) tested an optimization model for a hypothetical regional system, which improved the ability to handle unexpected challenges.

Using water for hydraulic fracturing, a method to extract shale gas and tight oil, has become increasingly controversial. This process requires large amounts of water and can create polluted water, which may worsen local water shortages and contaminate groundwater. As energy prices in North America and Europe have risen in the 2020s, interest from governments and industries in hydraulic fracturing is likely to increase.

Energy intensity

Urban water systems need a lot of energy to operate. Processes like moving water, using it, and cleaning wastewater use large amounts of energy. This has led to discussions about how much energy is used and how much carbon is released by water systems.

In 2001, water systems in the United States used about 3% of the country’s total electricity, which was roughly 75 terawatt-hours (TWh) per year. California’s State Water Project (SWP) and Central Valley Project (CVP) are the largest water systems in the world. These systems move water over 2,000 feet across the Tehachapi Mountains, from the wetter northern part of the state to the central valley and then to the dry, heavily populated southern region. The SWP and CVP use about 5 TWh of electricity each year. In 2001, 19% of California’s total electricity use, or about 48 TWh per year, was used for water-related tasks, with 65% of that coming from urban areas. In addition to electricity, 30% of California’s natural gas use was linked to water-related tasks, mostly for heating water at homes. About 88 million gallons of diesel fuel were used yearly by groundwater pumps for agriculture. The residential sector alone used 48% of the total electricity and natural gas used for water-related tasks in the state.

According to a report by the California Public Utilities Commission (CPUC) Energy Division, energy intensity refers to the amount of energy used in producing or moving water. In 2005, delivering water to Southern California had an average energy intensity of 12.7 megawatt-hours (MWh) per million gallons (MG), with nearly two-thirds of that energy used for transportation. After finding that about 20% of California’s electricity was used for water-related tasks, the CPUC started a statewide study on the connection between energy and water. This study, led by the California Institute for Energy and Environment (CIEE), helped create programs to save energy through water conservation.

The World Energy Outlook 2016 reported that in the Middle East, the water sector’s share of total electricity use is expected to rise from 9% in 2015 to 16% by 2040. This increase is due to more desalination plants, which produce fresh water from seawater. The Arab region includes many countries, such as Kuwait, Lebanon, Libya, Morocco, and others. This region covers about 10.2% of the world’s land area but only receives 2.1% of the world’s average yearly rainfall. It also holds only 0.3% of the world’s renewable water resources. Because of this, the region’s fresh water supply per person has been decreasing. By 2030, the water shortage is expected to triple, and by 2050, it will quadruple. This is a serious issue because the Arab region plays a key role in global economic stability.

To reduce the growing gap between water supply and demand, desalination is widely used, especially in the Gulf Cooperation Council (GCC) countries. About 50% of the world’s desalination capacity is in the Arab region, with most of it in GCC countries. For example, Bahrain gets 79% of its fresh water from desalination, Qatar gets about 75%, Kuwait about 70%, Saudi Arabia 15%, and the United Arab Emirates about 67%. These countries built large desalination plants to meet water needs because they have grown economically. However, agriculture in the GCC region, which makes up about 2% of the region’s GDP, uses 80% of the water produced. These desalination plants require a lot of energy, mostly from oil. If current trends continue, countries like Saudi Arabia, Bahrain, and Kuwait may struggle to meet future water needs. The GCC region uses 10–25% of its electricity to produce desalinated water.

Hydroelectricity

Hydroelectricity is a type of energy made from water. It is considered clean and renewable because it uses natural water sources. Dams, which are the main way to create hydroelectric power, have other uses besides making electricity, such as preventing floods, storing water, controlling water flow, and providing areas for recreation. These multiple uses make it difficult to fairly assess how resources should be used.

The effects of hydroelectric power on the environment are hard to measure. For example, water stored behind dams can lose more water through evaporation, and the water in reservoirs can be colder than water in rivers. Sometimes, managing water flow can create competition for water use over time, which also needs to be considered when studying environmental effects. People's willingness to pay for certain benefits can help estimate the cost of these effects.

One way to use water for energy is to retrofit existing dams to produce electricity. While using dams for energy is seen as a cleaner option than other energy sources, it can still harm the environment. Hydroelectric power is often considered a way to produce energy with fewer carbon emissions. However, recent studies show that dams can also release greenhouse gases. For example, a study by Galy-Lacaux et al. measured emissions from the Petit Saut Dam on the Sinnamary River in French Guyana over two years. The researchers found that about 10% of the carbon stored in the ground and plants was released as gas within two years.

Water Availability

As new renewable energy technologies are developed, they place extra pressure on available water resources. Technologies like biofuels, concentrating solar power (CSP), carbon capture and storage, and nuclear power require large amounts of water. A lack of available water can greatly affect how much energy is produced and how dependable it is.