Water and energy are fundamental components of 21st-century life, but they can no longer be considered separately. Just as producing energy consumes water, pumping, treating, and distributing water requires energy.
In other words, water is an energy issue, and energy is a water issue. This interrelationship, called the water-energy nexus, is beginning to receive the attention it merits.
Disruptions to the complex infrastructure that supplies society with these resources highlight their often invisible connections. A few cautionary tales from the news illustrate this point in stark terms:
In August 2003, a blistering heatwave swept through France, killing nearly 15,000 people. Dropping water levels and warmer temperatures in rivers severely limited the cooling water supply to nuclear power plants, which were forced to reduce electricity outputs just as demand for air conditioning spiked.
In October 2007, a prolonged drought brought Atlanta, Georgia, within months of running out of drinking water. Levels in Lake Lanier, which serves consumers and the Farley Nuclear Power Plant, dropped dangerously low, forcing complex choices between the supply of drinking water, the availability of electric power, and the survival of endangered species.
In September 2008, Hurricane Ike made landfall on the coast of Texas, shutting down 20 percent of the water systems in the Galveston area. One-quarter of backup generators stopped or ran out of fuel, and 14 percent of wastewater stations failed, discharging sewage into local rivers.
All three of Houston’s water pumping stations lost power, and officials warned residents to boil their water before drinking. Water and energy are intricately linked but have not always been managed as interrelated resources.
In May 2006, Energy & Environment Publishing began its report on a conference of experts meeting in Albuquerque, New Mexico, with this statement: “Water and energy maybe two of the South- west’s biggest natural resource issues, but few policymakers or resource managers consider the two together in making decisions about them, even as Western states scramble to meet skyrocketing demand for both.”
The Energy Policy Act 2005 represents the first time the federal government formally recognized the water-energy nexus. Section 979 directs the U.S. Department of Energy, in collaboration with other agencies, to “address energy-related issues associated with the provision of adequate water supplies” and “address water-related issues associated with the provision of adequate [energy] supplies.”
The resulting Report to Congress on the Interdependency of Energy and Water concluded that major changes in the generation, transmission, and distribution of energy might be needed in certain regions to address water issues.
The water-energy nexus can be considered from two main perspectives: the energy consumed to pump, treat, and deliver water and the water used to produce energy. Awareness of both perspectives is essential for resource management.
Water Consumes Energy
Water does not pour from the tap without first consuming power to get there. The electricity requirements for the delivery of potable water are enormous. By some estimates, 80 percent of the cost of water provision is related to energy.
Energy is required at every stage: extraction, conveyance, treatment, distribution, use, wastewater collection, treatment, and reuse or discharge.
On a national level, water and wastewater energy consumption account for as much as 4 percent of all the electricity produced annually. In other words, consumers exchange electric power for clean water supplies.
Groundwater Extraction
Groundwater accounts for 40 percent of Arizona’s water supply. The extraction of groundwater for potable use, on average, consumes 30 percent more electricity than diversions from surface water sources, primarily because of the pumping requirements.
In some areas of Arizona that rely almost exclusively on groundwater, the energy costs of such dependence can be high. Costs vary depending on the type of energy used, the groundwater depth, and the aquifer's physical characteristics.
The Arizona Department of Water Resources estimates groundwater prices range from $20 to $166 per acre-foot, representing varied energy requirements.
Groundwater depletion, a problem in several Arizona regions, can increase energy costs in several ways. Wells must be drilled deeper, and pumps lift the water higher. If water quality diminishes with the lowering of the water table, energy-consuming treatment is needed.
In some regions of Arizona, declining groundwater levels have increased the cost of pumping water for irrigation. This, along with development pressures, has led to some farmland ceasing production.
Surface Water Diversion and Transportation
Surface waters, such as the Salt, Gila, and Verde rivers, account for 56 percent of Arizona’s water supply. That includes the state’s single largest surface water source, the Colorado River on Arizona’s western border.
Capturing surface water often costs less than extracting groundwater, but energy costs are substantial when the water must be transported long distances from the diversion point.
The Central Arizona Project (CAP), Arizona's largest single electricity user, bears out this. A 336-mile system of canals, pipelines, and storage facilities, CAP delivers Colorado River water from Lake Havasu to its terminus south of Tucson.
Last year, CAP used approximately 2.8 million megawatt-hours of energy—about 4 percent of all the energy consumed in Arizona—to deliver 1.5 million acre-feet of water to central and southern Arizona.
All that power is needed to move water uphill: pumping plants lift water 2,900 feet over the canal's length. On average, CAP uses 5.5 kilowatt-hours of electricity for every thousand gallons (kgal) of water it delivers.
In other words, its energy intensity is 5.5 kWh/kgal. For comparison, a collaborative effort by the University of Arizona and Northern Arizona University researchers found that the energy intensity of potable groundwater pumped for the cities of Patagonia and Benson was 1.4 and 3.1 kWh/kgal, respectively.
That, however, is not the whole story because delivering water to Phoenix requires less energy than delivering water to Tucson, which is more than 100 miles farther and 1,400 feet higher in elevation. By one estimate, the energy intensity of CAP water delivered to
Tucson's energy consumption is 9.8 kWh/kgal, nearly double the system-wide average. CAP managers have long recognized the project’s energy needs. Lifting water 824 feet from Lake Havasu into the Hayden-Rhodes Aqueduct accounts for about half the power consumed in the entire CAP system.
Water is lifted at night to take advantage of lower, off-peak electricity costs and stored overnight in this large section of the canal.
Similarly, Lake Pleasant enables CAP to manage power costs on a seasonal basis. The lake is filled during the winter when energy is cheaper, and water is released in the summer when demand is high and energy is more expensive.
This attention to cyclic patterns in electricity prices reduces the energy costs of the canal’s operation, though not the total energy consumed.
Water Treatment
The water treatment process consumes energy in two ways. First, groundwater and surface water are treated before arriving at the tap. This does not normally consume a large portion of the total energy costs.
An Arizona Water Institute study led by Chris Scott and Martin Pasqualetti found that water treatment methods in Tucson require only a fraction of a kilowatt-hour per thousand gallons treated. Energy costs are higher for lower-quality sources—CAP water, for instance, requires more treatment than most groundwater.
At the other end of the domestic water cycle, treatment facilities receive municipal waste treated and then discharged or delivered for reuse. Wastewater treatment is more intensive than drinking water treatment because of the solids the wastewater contains.
Scott and Pasqualetti found that collecting and treating wastewater in Tucson requires about 1 kWh/kgal. Small rural systems often have higher energy intensities because of limited budgets or if they choose more intensive treatment technologies. In Benson, collecting and treating wastewater consumes 7.3 kWh/kgal, and in Patagonia, 13.5 kWh/kgal.
About 4 percent of Arizona’s annual water supply currently comes from wastewater treated for reuse or recharge (underground storage). Extended droughts and groundwater overdrafts raise costs and reduce groundwater and surface water supplies, so the importance of treated wastewater—long overlooked as a potential supply—is expected to grow.
Wastewater treatment plants can sell effluent for various uses, such as recharging aquifers, irrigating golf courses, and filling artificial lakes with further polishing to meet the Arizona Department of Environmental Quality standards.
Its potential uses expand, excluding direct contact with the drinking water system (see Arroyo 2009). Wastewater can be treated to a quality that matches its intended use, so the energy costs of its reclamation can be minimized.
It may even be possible to clean up wastewater while generating electricity rather than consuming it. When naturally occurring bacteria break down organic matter in wastewater, they release electrons.
Microbial fuel cells, an emerging technology, capture those electrons to create an electrical current. The technology is promising but still in the experimental stage. In the meantime, effluent is an ideal water source for generating electricity using conventional methods.
The Arizona Department of Water Resources encourages power plants to use effluent for cooling water. Effluent is the only water source that grows with population, so it is logical for generating stations (which also “grow” with population) to look to this resource to replace groundwater.
At least three generating facilities in Arizona—Palo Verde, Redhawk, and Kyrene—use effluent in their cooling towers, consuming more than 63 million gallons daily.
Overcoming the Dilemma
Undoubtedly, the water-energy nexus involves many tradeoffs, and the solutions to shortages are not always clear-cut. Officials know that water has energy and comes with water costs, which must be understood as dynamically linked. These links complicate planning and policymaking, as decisions that conserve one resource may negatively impact the other.
One thing is clear: recognizing the importance of the water-energy nexus is a critical first step toward a sustainable future.
As we search for new water supplies, we will find more distant and lower-quality sources, increasing demand for energy for transport and treatment. Likewise, the nation’s new commitment to developing alternative energy presents difficult water choices for dry regions like Arizona.
Shifting away from fossil fuels means a closer look at nuclear power, hydropower, and concentrating solar power —generally more water-intensive than coal or natural gas. Biofuel-run cars are cleaner but currently guzzle more water than gasoline, mainly if the crops are grown in dry regions.
Under normal circumstances—a rapidly growing population in an area with finite resources—the water-energy nexus seems like an unsolvable puzzle. Climate change only complicates matters, potentially reducing resources as the need for water and energy becomes more acute.
Yet inventive people across the state seek ways to simultaneously improve water and energy conservation. Guided by federal and state regulations, power providers are becoming more efficient, and cities are reconsidering their water sources and uses.
Researchers developing alternative energy have begun to recognize that water supply is intricately connected—either a cost to weigh or a potential benefit for which to strive. Individual consumers, too, can make meaningful choices as they consider water and energy interactions.
Switching off a light bulb can help preserve the state’s water supply, just as turning off the faucet represents savings in energy.
Understanding this nexus allows consumers to prioritize choices that have double benefits, like conserving heated water. Policymakers, scientists, and citizens all have a role in finding and adopting the win-win path to water and energy sustainability.
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