The Midwest is rapidly depleting its own water supply as well.
"About 27 percent of the irrigated land in the United States overlies the aquifer, which yields about 30 percent of the ground water used for irrigation in the United States. Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%. Depletion is accelerating, with 2% lost between 2001 and 2009 alone. Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall."
The core Midwest doesn't depend on that. The great plains do, but Iowa, Indiana, Illinois, etc. don't. You would probably get bored as fuck with corn and soy beans. But it would v feed tons of people if we didn't feed it to cows or gas tanks.
"About 27 percent of the irrigated land in the United States overlies the aquifer, which yields about 30 percent of the ground water used for irrigation in the United States. Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%. Depletion is accelerating, with 2% lost between 2001 and 2009 alone. Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall."
http://en.wikipedia.org/wiki/Ogallala_Aquifer