W ater and energy are critical resources that are reciprocally and mutually linked. Meeting energy needs depends upon the availability of water, often in large quantities, for mineral extraction and mining, fuel production, hydropower, and thermoelectric power plant cooling. Likewise, energy is required for the pumping, conveyance, treatment and conditioning, and distribution of water and for collection, treatment, and discharge of wastewater. This interdependence, which is often described as the water-energy nexus, or energy-water nexus, is illustrated in the following graphic from a U.S. Department of Energy report. Figure 1. Examples of Interrelationships Between Water and EnergySource: U.S. Department of Energy, Energy Demands on Water Resources, Report to Congress on the Interdependency of Energy and Water, December 2006, p. 13. This report first discusses water-related energy use broadly and then energy for facilities that treat and deliver water to end users and also dispose of and discharge wastewater. There is growing recognition that "saving energy saves water," and the report describes options and impediments for energy efficiency by these facilities. It also identifies several areas of research and information needs concerning energy for water uses. In the United States, more than 400 billion gallons of water are withdrawn daily from surface and ground water sources of freshwater and saline-water to supply domestic uses, agriculture including irrigation, industry, mining, and thermoelectric power. Information about the energy—especially electricity—that is needed to pump, transport, deliver, and process that water is fragmentary and not well documented overall. In particular, as described further below, energy needs for self-supplied domestic, industrial, and energy water are largely unknown, but are likely to be large. Interest has been growing in better understanding of the energy-related needs of providing water to diverse sectors of the economy. In a 2002 report, the Electric Power Research Institute (EPRI) estimated that nearly 4% of the nation's electricity use goes toward moving and treating water and wastewater by public and private entities. EPRI's analysis covered public water supply agencies and publicly owned wastewater treatment facilities (accounting for 1.6% of U.S. electricity consumption in 2000) and self-generated private water and wastewater treatment (i.e., for industry and mining, agriculture, and commercial supply and treatment, accounting for 2.1% of U.S. electricity consumption in 2000). EPRI projected that electricity consumption by these sectors would increase by 23% in 2020 from 2000 levels and by 63% in 2050. Today that 4% estimate is considered a good starting place for understanding the magnitude of energy demands for providing these water services, but deficient in several respects. It relied on secondary source data. It did not include future projections of electricity requirements for water supplies in the thermoelectric sector (because it assumed that energy for water use in this sector would decline). It did not consider on-site heating, cooling, pumping, and softening of water for end-use. It did not consider that in the future a large proportion of new water demands will be met by sources with greater energy intensities, such as groundwater pumped from greater depths and seawater desalination. Others have attempted to expand on the EPRI analysis, using additional and updated data from a variety of sources, to develop a baseline estimate of water-related energy use in the United States. For example, a 2009 report by the River Network, a national advocacy group for freshwater conservation and watershed restoration, looked at data from the Energy Information Administration (EIA) concerning U.S. energy use for residential and commercial water heating, which comes from multiple fuel sources (e.g., electricity, natural gas, fuel oil). It found that more than 380 billion kilowatt hours (kWh) of energy is used for water heating by these sectors; 148 billion kWh, or nearly 40% of that energy, is supplied by electricity. Researchers at the University of Texas at Austin have attempted to quantify the energy embedded in the U.S. public water supply, which is the primary water source of residential, commercial, and municipal users. One such analysis concluded that energy use associated with the public water supply is 4.1% of the nation's annual primary energy consumption and 6.1% of national electricity consumption, but this analysis excluded energy requirements associated with water for agriculture, industrial, and self-supplied sectors (e.g., agriculture, thermoelectric, and mining). In this analysis, electricity consumption by public drinking water and wastewater utilities for pumping, conveyance, treatment, distribution, and discharge was 56.6 billion kWh, or 11.5% of primary energy and 21.6% of electricity consumption for water end-use, respectively, in 2009. A second analysis by these researchers looked more broadly at energy needs for water supply, adding industrial and thermoelectric sectors to others considered previously. Water-related energy use throughout the economy varies across sectors, and analysis of some sectors is complex and limited by incomplete data (e.g., cooking-related activities vary across residences and are not well documented). This analysis concluded that direct water-related energy consumption was 12.6% of national primary energy consumption in 2010. This amount of energy, 12.3 quadrillion BTUs, is the equivalent of annual energy consumption of about 40 million Americans. It also estimated that energy losses at the point of electricity generation, transmission and distribution, and end-use represent 58% of the total primary energy that was consumed for water-related purposes, reflecting varying efficiencies of water heating and boiler technologies. The estimate of waste heat losses is subject to uncertainty, the researchers said. Because of inadequate data and other factors, missing from some analyses is water-related energy for several important end-use sectors. Self-supplied water, which is a high percentage of power plant use and of some industrial uses such as mining. Some of this energy-for-water is in the form of electricity, but much of it is likely direct use of fuels on-site. Privately operated residential water supply wells also utilize energy for pumping, none of which is accounted for. Agricultural use of water for livestock and irrigation—second in volume only to water use for thermoelectric power, according to the U.S. Geological Survey—is generally omitted from these analyses, although substantial energy, which generally is self-supplied, is needed for pumping. The transportation sector, although the majority of energy consumed is for petroleum-based transportation fuels, which is presumably reflected in water-for-energy analyses. The bottled water industry, which is a substantial drinking water source in the United States and consumes energy to collect, treat, bottle, and distribute its products. Energy use for water is a function of many variables, including water source (surface water pumping typically requires less energy than groundwater pumping), treatment (high ambient quality raw water requires less treatment than brackish or seawater), intended end-use, distribution (water pumped long distances requires more energy), amount of water loss in the system through leakage and evaporation, and level of wastewater treatment (stringency of water quality regulations to meet discharge standards). Likewise, the intensity of energy use of water varies depending on characteristics such as topography (affecting groundwater recharge), climate, seasonal temperature, and rainfall. National data can obscure differences in water-related energy use that are regional or state-specific, as reflected in a 2005 study by the California Energy Commission, which found that "water-related energy use [in California] consumes 19 percent of the state's electricity, 30 percent of its natural gas, and 88 billion gallons of diesel fuel every year—and this demand is growing." Pumps that move water from the San Joaquin Valley to southern California for domestic and irrigation water uses are the single largest power load in the state. Because of important regional differences, the United States is a difficult country to generalize. For example, the lifecycle energy-intensity of water in cities nationally is estimated to be 3,300-3,600 kWh per million gallons delivered and treated, but ranges from 2,700 kWh/million gallons in New York, New York, to 5,000 kWh per million gallons in Austin, Texas. Energy intensity varies within states, as well. In California, the energy intensity of the water use cycle ranges from 4,000 kWh per million gallons in the northern part of the state to 12,700 kWh per million gallons in southern California, reflecting differences in the volume of water pumped, lifted, and transported hundreds of miles and over mountains from points of collection to points of need in the southern part of the state. The energy intensity of a particular activity's water use, also described as the embedded energy of the activity, can have disproportionate impacts elsewhere. For example, policies that promote the use of energy-intensive water supply such as pumping and distributing water over long distances, rather than policies that promote water conservation, water reuse, or aquifer recharge, adversely impact one sector to serve another. In every sector, there are opportunities for practices that would save energy and also save water. The Environmental Protection Agency's (EPA's) WaterSense program promotes this concept by emphasizing that "saving water saves energy." Energy efficiency initiatives offer opportunities for delivering significant water savings, and likewise, water efficiency initiatives offer opportunities for delivering significant energy savings. In the commercial, industrial, and institutional sectors, potential water savings through energy efficiency and other measures could be 15%-30% without reducing the services derived from the water. The potential for significant water and energy savings also exists in other sectors such as agriculture. Generating the energy associated with water use also produces carbon dioxide (CO 2 ) emissions that contribute to climate change. It has been estimated that water-related carbon emissions in 2005 were approximately 290 million metric tons, or 5% of all U.S. carbon emissions. Water-related CO 2 emissions were equivalent to the annual greenhouse gas emissions of 53 million passenger vehicles.