California likes to think of itself as being ahead of the curve. So when the state set out to reduce greenhouse gas emissions, regulators did all the right things - stringent tailpipe standards for cars, tighter codes for buildings, higher renewable energy standards for utilities. Then they took one of the most aggressive energy-saving steps of all.

They started a campaign to save water.

The link between energy and water is not always apparent, but the two are as intertwined as the hydrogen and oxygen atoms in a bottle of Evian.

By now, everyone knows you save energy by turning out lights. And you conserve water by taking shorter showers. But it's just as true that saving water may be one of the most effective ways to save energy - and vice versa. "It's a 'buy one, get one free' deal," said Douglas Kenney, a professor at the University of Colorado Law School and the editor of an upcoming book that explores the nexus of water and energy.

In California today, just delivering water accounts for 20 percent of the state's energy consumption. It takes power to gather water, purify water, and distribute water, especially in places like southern California where water is piped hundreds of miles to supply Los Angeles' sprawling demands.

Nationally, energy production sucks more water from freshwater sources than any other sector except agriculture. It takes water to create the power we use to drive our cars, transport our groceries, and run our toaster ovens. Virtually every source of electricity in a typical American home or manufacturing plant - whether it comes from hydroelectricity, coal, natural gas, nuclear, biofuels, or even concentrated solar -- also requires water. Lots of water.

That's a growing problem, because in many places, finding water for energy isn't easy - and it's bound to get tougher as energy demands soar and climate change alters hydrological cycles in already arid regions. The energy sector is the fastest-growing water consumer in the United States, according to a January 2011 Congressional Research Service report.

Nationally, that's a challenge, but regionally it could be a calamity. As the Congressional Research report notes, "much of the growth in the energy sector's water demand is concentrated in regions with already intense competition over water."

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The connection between energy and water - and the precariousness of that link in the western United States - is exemplified in a gigantic plug of concrete stopping the muddy Colorado River above Las Vegas, otherwise known as Hoover Dam. At the ceremony inaugurating the Depression-era public works project in 1935, then-Interior Secretary Harold Ickes noted proudly, "no better understanding of man cooperating with nature can be found anywhere."

Hoover Dam provided the two key ingredients - water and power - that freed the Southwest and southern California to go on a 75-year growth spurt. Lake Mead now supplies water to more than 22 million people, and it produces more than four billion kilowatts of electricity per year.

But Ickes likely never imagined how quickly man's cooperation with nature would disintegrate in the 21st century. In the American West, a burgeoning population created a double-whammy of surging power demands and dwindling freshwater supplies. The Colorado River, lifeblood of seven western states, is already as overdrawn as the federal treasury. Drought conditions during most of the 21st century have forced water managers to plan for a day when the region's vast system of dams and reservoirs no longer have enough water to store. Already, utilities have to scramble to respond on days when everybody in Phoenix, Las Vegas and Los Angeles wants to crank their air conditioners during the same heat wave.

Sustained drought and insatiable upstream water demand have drained Lake Mead to the point that experts are predicting it may soon be shallow enough to be in deep trouble. Despite near record snowfalls and runoff this year that raised its level from historic lows in January, Lake Mead is still 113 feet below "full pool" - and is filled to less than 50 percent of its capacity.

Three years ago researchers at Scripps Institution of Oceanography warned Lake Mead has a 50-50 chance of running dry by 2021 and that the reservoir's water level could dip low enough to reduce or stop electricity production as early as 2013. Although this year's run-off probably forestalled this dramatic assertion, utilities around the country have already been forced to reduce or stop electrical production because of water issues. According to a survey done in California's 2009 Water Plan Update, states from Virginia to Nevada and Texas to North Dakota have all curtailed energy development projects because of water quality or quantity concerns.

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One reason for this problem is that electricity, as we've chosen to produce it, is pretty wet stuff. Plug an appliance into an outlet and you might as well open a faucet as well. Running an average refrigerator all day uses about as much water as a ten-minute shower (without a low-flow showerhead). According to the U.S. Geological Survey, electric power generation accounts for nearly half of the nation's water usage; it takes on average 21 gallons of water to produce one kilowatt hour of electricity. In the arid West, those numbers add up. A report by Western Resource Advocates notes that "thermoelectric power plants in Arizona, Colorado, New Mexico, Nevada, and Utah consumed an estimated 292 million gallons of water a day in 2005 - approximately equal to the water consumed by Denver, Phoenix, and Albuquerque, combined."

Pretty much every step of energy production requires water, from mining to refining, processing to generation. Some of this water is "consumed" - evaporated as steam. Some of it is returned to watersheds in altered forms - like water heated during coal-fired electrical production and stored in cooling towers or ponds before being released - at higher temperatures - back into rivers. "Produced" water from coal-bed methane extraction releases underground water with high mineral content into watersheds. Deep drilling for seams of underground gas deposits rely on chemicals used in "fracking fluids," which contaminate water sources when they leak.

Other potential fossil fuel energy sources, like oil shale, require so much water during its production cycle that energy companies in Colorado have stealthily acquired rights to develop hundreds of thousands of acre feet of water, even before they've invented a viable technology to turn that rock into oil. An acre foot of water is 325,851 gallons, or enough to cover an acre of flat farmland with water a foot deep.

That's enough water to escalate the state's already intense water disputes into open warfare. "If oil shale energy does become commercially viable, it will be a huge new water drain," says Dan Luecke, a Colorado-based hydrologist and Western water consultant.

Many current energy debates have focused on the massive carbon footprint of fossil fuels like oil, coal and natural gas. But many renewable sources of energy, like corn-based ethanol, have a huge and potentially troubling "water footprint." Corn ethanol made from irrigated crops, for example, can use more than 1,000 times more water than oil refining, according to calculations by Sandia National Laboratory. Industrial concentrated solar arrays can require 800 gallons of water to produce a single megawatt hour. Mike Hightower, a senior researcher at Sandia National Laboratories in New Mexico, cautions that reducing carbon emissions, while crucial, is just one part of the energy equation: Virtually every time you lower the carbon footprint in industrial energy production, he says, "you end up with a bigger water footprint."

As planners look to the future, they have to grapple with some tough trends: the more energy we need, the more water we need. But the availability of fresh water has already reached crisis proportions in many parts of the world, and some experts warn we should be more worried about "peak freshwater" than "peak oil." According to Peter Gleick and Meena Palaniappan, writing in the Proceedings of the National Academy of Sciences, water availability is a growing global problem, especially in regions like the Western U.S. where "almost all major rivers and aquifers and already tapped out." Unlike oil, they write in dry, understated concern, water is absolutely essential for life. "For many uses," they conclude, "it has no substitutes."

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Climate change is only going to make the water-energy balance more precarious.

Arid mid-latitude regions like the West are warming nearly twice as fast as the global average, according to the Rocky Mountain Climate Organization. As the West warms, residents will need more energy to cool living spaces and make desert cities like Tucson and Scottsdale inhabitable - and will likely have less water to make enough electricity to do that.

The collision of water, energy and climate change will reverberate through public policy decisions for decades to come, with unintended consequences at each step. Congress effectively encouraged a giant sucking sound from Midwestern aquifers and rivers by creating massive subsidies for corn ethanol. Concentrated solar projects, which have received "fast-track" authority from the Obama administration, may run into water problems before the first watts are generated. Citizen opposition to new coal-fired power plants in places like Nevada and Montana has focused as much on water concerns as greenhouse gas emissions.

Global climate models predict that arid regions of the world will become more arid as a result of rising greenhouse gas emissions. According to the U.S. Global Change Research Program's report, Global Climate Change Impacts in the United States, a one percent decrease in precipitation leads to a two to three percent drop in stream flow. Every percentage point that stream flows drop means a three percent decline in electricity generation. The report's conclusion is as obvious as it is ominous: "Water and energy are tightly interconnected."

Some energy sources, like rooftop photovoltaics and most wind power, are not water hogs, but experts say they are not likely to fill the nation's growing power needs by themselves. Conservation - both of water and of energy, are undeniably going to be part of any future plan, as are technological improvements in wastewater treatment and reclamation. "People are beginning to understand that if you save water, you save energy," says Sandia National Laboratory's Hightower.

They also need to also understand that if they save energy, they'll save water as well. Which, in the long run, may be an even more important thing to conserve.

Daniel Glick, a former Newsweek correspondent, is co-founder of the Story Group (http://thestorygroup.org/) with photographer Ted Wood. DailyClimate.org is a nonprofit news service covering climate change.

On the web:

Congressional Research Service report on water consumption (pdf)

Lake Mead water data

California's 2009 Water Plan Update (pdf)

USGS report on water and energy (pdf)

Western Resource Advocates report on shale water rights in Colorado (pdf)

Peter Gleick, Meena Palaniappan PNAS paper

Rocky Mountain Climate Organization report (pdf)

US Global Change Research Program report

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.