The researchers said agriculture costs the water supply in two ways: through the drawdown of groundwater from irrigation and through loss to the atmosphere via evapotranspiration (ET), by which water moves through plants and evaporates. Higher atmospheric temperatures increase ET at a cost to groundwater, they wrote.
The production of one liter of gasoline requires three liters of water, according to the researchers. The production of one liter of corn ethanol requires between 350 and 1,400 liters of water from irrigation, depending on location. A liter of ethanol also translates into 1,600 liters of ET water that might not directly replenish the local watershed.
The researchers suggested the growth of crops for ethanol was already questionable because of its impact on the environment. Rising temperatures in the decades to come, they wrote, could lead to reductions in crop yields and an increase in irrigation demands to the degree that the government mandate is no longer economically viable.
“The projected increases in water intensity due to climate change highlight the need to re-evaluate the corn ethanol elements of the Renewable Fuel Standard,” Dominguez-Faus said.
Dominguez-Faus, lead author of the paper, is a postdoctoral researcher at the University of California at Davis. Co-authors are Christian Folberth of EAWAG Aquatic Research, Dübendorf, Switzerland; Junguo Liu, a professor at the School of Nature Conservation, Beijing Forestry University; and Amy Myers Jaffe, executive director of energy and sustainability at the UC Davis Institute of Transportation Studies.