All corn producing regions are not equal. Some land is less suitable for growing corn, which is a thirsty crop and hungry for expensive inputs. As the above infographic shows, in some regions precious aquifer water is being pumped unsustainably to grow corn.
Two points policy needs to consider:
1) In some cases this irrigated corn is being converted to ethanol, and then exported to far reaching corners of the globe.
2) It requires energy to pump water out of the ground, so the small net energy gain claims of corn ethanol turn negative when irrigation is used to grow that corn.