Water and energy are fundamental components of our 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; energy is a water issue. Called the water-energy nexus, this interrelationship 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 supply of cooling water 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 as well as 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, taking 20 percent of water systems in the Galveston area out of service. Fully one-quarter of back up generators stopped or ran out of fuel, while 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 it. Water and energy are intricately linked, but they 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 of 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 points of view: energy consumed to pump, treat, and deliver water and 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 on an annual basis. In other words, consumers exchange electric power for clean water supplies.
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 depth to groundwater, and the physical characteristics of the aquifer. The Arizona Department of Water Resources estimates groundwater prices range from $20 to $166 per acre-foot—varied prices that represent varied energy requirements. Groundwater depletion, a problem in a number of Arizona regions, can increase energy costs in several ways. Wells must be drilled deeper, and the water itself must be lifted higher by pumps. If water quality diminishes with the lowering of the water table, this creates a need for energy-consuming treatment. In certain areas of Arizona, groundwater decline has caused the cost of pumping water for irrigation to rise. Combined with development pressures, this has resulted in some farmland going out of 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 when the water must be transported long distances away from the diversion point, energy costs are substantial.
This is borne out by the Central Arizona Project (CAP), the largest single user of electricity in Arizona. 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 length of the canal. 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 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 oversized 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.
The water treatment process consumes energy in two ways. First, groundwater and surface water is 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 that must be 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. Currently, about 4 percent of Arizona’s annual water supply comes from wastewater treated for reuse or recharge (storage underground). Extended droughts and groundwater overdraft necessarily raise costs and reduce supplies of groundwater and surface water, so the importance of treated wastewater—long overlooked as a potential supply—is expected to grow. Wastewater treatment plants can sell effluent for a variety of uses—recharging aquifers, irrigating golf courses, filling artificial lakes. With further polishing to meet the Arizona Department of Environmental Quality standards, its potential uses expand, excluding only direct contact with the drinking water system (see Arroyo 2009). Wastewater can be treated to a level of 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 source of water for generating electricity with 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 as a replacement for groundwater. At least three generating facilities in Arizona—Palo Verde, Redhawk, and Kyrene—use effluent in their cooling towers, consuming a total of more than 63 million gallons a day.
Overcoming the Dilemma
Undoubtedly, the water-energy nexus involves many tradeoffs, and the solutions to shortages are not always clear-cut. Officials are now aware not only that water has energy costs and energy comes with water costs, but also that these costs must be understood as dynamically linked. These links complicate planning and policymaking, as decisions that conserve one resource may have detrimental impacts on the other. One thing is clear: recognizing the importance of the water-energy nexus is a critical first step toward a sustainable future. As our search for new water supplies takes us to more distant and lower-quality sources, energy for transport and treatment will be increasingly in demand. Likewise, the nation’s new commitment to developing alternative energy presents difficult water choices to dry regions like Arizona.
Shifting away from fossil fuels means a closer look at nuclear power, hydropower, and concentrating solar power —all three generally more water-intensive than coal or natural gas. Biofuel-run cars are cleaner, but currently guzzle more water than gasoline, particularly if the crops are grown in dry regions. Under normal circumstances—a rapidly growing population in a region with finite resources—the water-energy nexus seems like an unsolvable puzzle. Climate change only complicates matters, potentially reducing resources just as the need for water and energy becomes more acute. Yet inventive people across the state are seeking out ways to make simultaneous gains in water and energy conservation. Guided by federal and state regulations, power providers are becoming more efficient, and cities are reconsidering their sources and uses of water. 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 the interactions of water and energy. Simply 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.