updated 3/10/2008 5:43:27 PM ET 2008-03-10T21:43:27

As environmentally friendly as hybrid and fully electric cars are, it turns out replacing normal vehicles with them might dangerously strain already scarce water reserves.

Plugged-in hybrid electric vehicles run on electric mode for a limited distance before they switch to an internal combustion engine for longer trips, while fully electric vehicles operate solely off batteries. Both are presumed to be better for the planet than normal vehicles, because they release fewer emissions into the air.

But these hybrid and fully electric cars rely in part on water. Specifically, the power plants that produce the electricity typically use water primarily to cool down the systems.

Such water consumption might be especially of concern in the United States "in the Southwest and Southeast and the West, where water resources are definitely strained," said researcher Michael Webber, a mechanical engineer at the University of Texas at Austin.

Webber and colleague Carey King compared the amount of water used, withdrawn and consumed during petroleum refining and electricity generation in the United States. They estimate that plugged-in hybrid and fully electric vehicles could sharply increase the country's water consumption, with each mile driven with electricity demanding roughly three times more water than gasoline.

The researchers note these concerns do not necessarily mean electric cars are undesirable. "It just means there might be some tradeoffs," Webber said.

Policymakers may want to move to less water-intensive cooling technologies, such as air cooling. Seawater or recycled former wastewater unsuitable for drinking might also help cool power generators. Power generators that use little to no water, such as wind or solar technology, might also find use, Webber noted.

Webber and King are scheduled to detail their findings in the June 1 issue of the journal Environmental Science & Technology. The study was done without any outside funding.

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