As energy visionaries contemplate greener routes for urban commutes, discussions frequently turn to electric vehicles — either gasoline-electric hybrids or dedicated electric-only cars. Sure, they’ll save on consumption of those precious fossil fuels and cut emissions of noxious air pollutants. What doesn’t usually enter into the discourse, however, are the likely trade-offs in another essential resource: water.

And big trade-offs there’ll be, according to researchers at the University of Texas at Austin.

Carey W. King and Michael E. Webber tallied the water used to extract and refine oil. It comes to about a third of a gallon of water per vehicle mile traveled by gasoline-powered vehicles in the United States, they report in paper posted early online Feb. 20 in Environmental Science & Technology.

For comparison, they summed the water used to mine fossil fuels or uranium and then generate electricity. The latter fuels are used to generate almost 90 percent of U.S. electricity. The Texas duo calculated that plugged-in vehicles would consume almost three times more water per mile traveled than gasoline-powered ones. Each electric-car mile would also require the withdrawal of 17 times more water than a mile traveled using gasoline.

Water withdrawal refers to water taken from a lake, stream or other source, then used to cool electric-generating equipment. Because this water is eventually released to the environment, it’s not actually consumed, but this water’s availability is required to provide electrical power and to perform key industrial processes.

A switch from gasoline- to electric-powered vehicles could threaten local water resources, the authors argue. Indeed, they conclude: “Some relatively wet regions of the United States may be able to support more plug-in hybrid electric vehicles at lower cost than other relatively dry regions.” Think the arid West and southeastern states.

There are ways to get around the problem … somewhat. The Texas researchers recommend investing more in research on renewable energy for onsite recharging of electric vehicles; developing regional water-use plans that account for the growing water need of socially desirable electric transportation; and using reclaimed, saline or other water unsuitable for drinking for cooling equipment.

Bottom line: We Prius owners must not get smug. Resource economists remind us there are always trade-offs.

Water-cooler stats

• in 2005, U.S. motorists drove an estimated 2.75 trillion miles (57 percent in cars, the rest in vans and SUVs)
• that’s 80 billion miles per year more than just 2 years
• it took 4.6 million barrels of oil per day in 2005 to meet the demand for gasoline to fuel U.S. cars and another 4.3 million barrels of oil to power the nation’s vans and SUVs
• distance the average U.S. motorist drives on a weekday: 34.4 miles — or 20 percent more than on a weekend day
• up to 2.5 gallons of water are consumed to refine each gallon of gasoline
• domestic gasoline refining would have consumed between 125 and 316 billion gallons of water in 2005
• U.S. consumers buy 17 million vehicles per year (cars, vans and SUVs)