I am mostly unbaware of the ethanol advantages or disadvantages but certainly depleting ground water to grow more corn would not be a good idea. What about sugar (beet) cane? Does growing cane or beets need as much water I wonder?
One thing for sure (?): we need to become energy independent.
You can grow sugar cane in the US in Florida and Louisiana. I'd say turn the New Orleans 9th ward into sugar cane fields. No shortage of water and a much better use of the land than government housing that will only flood again.
The term used by agriculturalists is Crop Consumptive Use. For Myrddin's area in Idaho (see post #29) the comparison between crops can be found here:
In Idaho, alfalfa hay requires 3.12 ft. of water, sugar beets 2.81, and potatoes 2.27 ft. Other areas of the country have different requirements because rainfall and temperature play a part. In general, the hotter the area, the more water is required for the crop. The drier the area, the more supplemental water must be applied.
So if you are going to grow a crop for this purpose, you should grow it in an area that has plenty of natural water that is replenished seasonally or annually.
Do a Google under "crop consumptive use" for publications in other areas of the country.