I am frequently asked, "How much water does it take to make a gallon of ethanol?" I respond, "Yes, and there are a myriad moving parts, and approximations, too."
First, we have to know what kind of ethanol we are talking about and where was it refined.
Most in U.S. use corn as a source, versus the world, where sugar cane is used. Most of our ethanol is made in Iowa, and E-15 and E-85 represent the percentage of ethanol in the gas.
A quick and dirty answer to our question might be the way the citizens of Burley, Idaho, analyzed the question of constructing an ethanol plant in their town; simply by dividing the water used at the plant by the gallons of ethanol produced. When that number came to 1/8 of the town's daily total water supply, or 3 gallons of water for every gallon of ethanol; they simply squelched the plant.
However, they neglected to consider the amount of water to raise the corn used by the plant. With 50 acres needed to grow corn for a gallon of ethanol, we calculate that it would take 5,480 acre feet of water to grow the corn, which doesn't appear high, unless you realize that equals 1.7 billion gallons of water, or 75 more gallons of water per gallon of ethanol.
The annual fuel consumed in the U.S. (2010) was 134 billion gallons, and the amount of water and corn to produce that amount of fuel would be staggering. A personal story is worth shedding some light here.
As luck would have it, I was always put in advanced classes at college and met some sharp cookies. One man became director of the CIA, and later head of the chemistry department at a leading university. He wrote an op/ed for a N.Y. newspaper announcing that it would take half the acres in America's heartland to produce enough corn to make ethanol for us to be about 50 percent energy independent, even planting corn in church parking lots and on little league baseball fields.
I contacted him, retrieved the acreage numbers, and applied water needs. It was staggering, akin to being offered the choice between death by sword or death by firing squad; as we would consume either several Mississippi Rivers, drying up most of The Great Lakes, irritating Canada no end; or we would use more than a dozen Colorado Rivers.
Ethanol is a green favorite, where corn farmers receive hundreds of billions in subsidies, but few consider its BTU value, its higher smog production (1995 third circuit court of appeals, API vs. EPA), or its chemistry. Few ecology marchers know that ethanol is so corrosive that it can't be shipped by pipeline, and the added cost of tank truck distribution is never factored into an efficiency equation. Ethanol produces 25% less energy (BTU) per volume than gasoline, and no one cares that every bushel of corn used for ethanol is one less bushel to feed cattle or starving folks across the globe.
As true of all things now; the Washington wizards know it all, know how to issue thousand page mandates, careful to the penny, careless to the trillion, and clueless to the end.
When you hear folks extoling ethanol, remember the facts; but as my Wyoming rancher friend says, "Why bother locking the gate after the horses are stolen?"
Jack Flobeck is the founder of Aqua Prima Center, a nonprofit think tank for water research. Readers can contact him at firstname.lastname@example.org.