If you learned that someone had invented a car that runs on water, you’d probably be thrilled. But if you found out that the car consumes 50 gallons of water for every mile driven, you might wonder if it’s worth it.

Of course any vehicle that requires 50 gallons of any liquid fuel is a nonstarter given the volume and weight of the fuel, but for the purposes of this thought exercise the issue is using up all that water.

Something very like this scenario is rapidly becoming a reality, and is even mandated by law. It turns out that producing ethanol from corn uses an awful lot of water, and the Energy Independence and Security Act of 2007 requires the U.S. to produce 15 billion gallons of corn ethanol annually by 2015.

A study by researchers from Rice University, Clarkson University and Missouri University of Science and Technology found that it takes 500 to 4,000 liters of water to grow feedstock to produce one liter of ethanol, depending on the crop and where it’s grown.

Given an 800-to-1 water-fuel ratio and a car that gets 16 miles per gallon of ethanol (ethanol has a lower energy density than gasoline, which means lower mileage), you’d use 50 gallons of water per mile. This is the case for Nebraska-grown corn. You’d use 23 gallons per mile for Iowa corn and 115 gallons per mile for Texas sorghum.

The 15 billion gallons of corn ethanol mandated by 2015 is only about 10 percent of the transportation fuel the U.S. is likely to use that year, but producing it will require the equivalent of 44 percent of the corn grown in the U.S. in 2007. Agriculture today accounts for 80 percent of the water consumed in the U.S., and our freshwater supply is already under a lot of pressure.

The water-use scenario is very different for cellulosic feedstocks, particularly drought-resistant plants like miscanthus that require far less water. In theory, many types of grasses can be grown without any irrigation.

This makes efforts to come up with economical and scalable cellulosic biofuel production all the more urgent. Sorting out the land-use issues around biofuels is challenging enough without worrying about water.

Eric Smalley is the editor of Energy Research News. He has written about technology since 1987 and has freelanced for many publications including Discover, Scientific American, Wired News and The Boston Globe on topics ranging from quantum cryptography to global warming.