Ask any grade-school student what a plant needs to grow, and he or she will likely mention two things: sunlight and water. While the first is a bit beyond the control of biofuel producers, the second is at the core of a potentially serious drawback of plant-based fuels: The water demands of some biofuel-producing crops could put unsustainable pressure on local water resources if not managed wisely.
A 2009 study suggests that, in the rush to produce enough corn-based ethanol to meet federal alternative energy requirements, biofuel demand is already putting stress on fresh water supplies in the Great Plains and central Southwest [source: McKenna]. Central to the problem is corn's relatively high water requirement. Researchers are investigating ways to genetically engineer less thirsty crops, and carefully planning what biofuel crops to plant in a given region can mitigate this problem [source: Lau]. But large-scale biofuel production -- especially using corn, and in arid parts of the world -- will have to share finite water resources with drinking and irrigation needs.