Wanna Save Water? Try Carpooling.

Water probably is not the first thing that comes to mind in thinking about making cars and trucks move. Until, that is,  you look at the amount of water needed to refine oil, produce biofuels, or cool power plants that will be needed to meet the demand for electricity as more electric vehicles hit the roads.

That's what Carey King and Michael Webber have done. The bottom line, according to the University of Texas researchers: As the United States tries to end what President Bush once referred to as the country's addiction to oil, the country likely will be trading foreign oil for domestic water. That's not a bad thing, they argue -- as long as people realize that rising water demand from the energy sector could have a significant effect on regional water supplies and plan accordingly. Energy represents one more set of users joining thirsty urban residents and farmers at the water trough.

Welcome to the water-energy nexus -- another arena where water supplies (or the lack of them), climate change (and efforts to reduce greenhouse-gas emissions), and rising populations (especially in already water-stressed regions) collide.

Much of the water-energy discussion has centered on power generation. Hydroelectricity is one obvious example. As the climate warms, for instance, winter precipitation in the US West is expected to shift from heavy seasonal snow in the Cascades, Sierras, and Rockies to winter rainfall. High-altitude snow -- the last to melt during the spring and summer -- represents late-year storage that vanishes. This reduces the water available for reservoirs behind the dams that have provided the region with abundant and cheap electricity. Elsewhere, vanishing alpine glaciers, which feed rivers that in turn drive generators, are retreating, further threatening long-term electricity supplies.

Hydropower dams aren't the only electricity generators affected by low water. In August 2007, the Tennessee Valley Authority shut down one of three reactors at its Browns Ferry nuclear plant. The plant draws from and returns its cooling water to the Tennessee River. Drought shrank water levels in the river. And the hottest temperatures in some 50 years (plus water warmed by power plants upstream) heated the available water to levels that would have topped permissible limits at the outflow pipe if the plant had continued to run all three reactors. Consumers already were bracing for rate increases because the low flow in the Tennessee's tributaries had cut the mount of juice hydropower plants in the system could produce.

Coal plants also need cooling. Some utilities are experimenting with approaches to cooling that try to stretch every gallon of water as far as possible. Or they are looking to salty underground water sources that are unfit for people but fine for power plants. Schemes for carbon capture and sequestration -- scrubbing CO2 from a power plant smoke stack and storing it underground -- also require water.

Drs. King and Webber opted to look at transportation demands for light cars and trucks in the US.  They looked at both water withdrawal (water drawn from the system and later directly returned) and water consumption (water drawn and not directly returned). They made their calculations for a range of fuels -- from traditional petroleum-based options such as diesel fuel and gasoline, to biofuels and electric vehicles. The numbers include water used for mining, farming, or drilling. Then they calculated the water consumption in gallons for each mile traveled using these other fuels.

Not surprisingly, while gasoline  consumes water at 0.15 gallons per mile, biofuels like ethanol slurp an average of 28 gallons per mile. Biodiesel from soybeans comes in at 8 gallons per mile. Electric cars and plug-in hybrids compare favorably with conventional fossil fuels, consuming a bit more than 0.2 gallons of water per mile. The most watertight combo: electric vehicles recharged through renewable sources of electricity. The study appeared in the Nov. 1 issue of Environmental Science and Technology.