“Corn is a huge user of water.”
A lot of people forget that most of the agriculture west of about central Nebraska is dependent on irrigation, and a lot of that comes out of the aquifers, not from rivers.
Total water use for irrigating crops is actually trending down.
http://water.usgs.gov/edu/gwdepletion.html
The Ogallala aquifer in the midwest (Kansas, Nebraska, etc.) has for a long time now had more water taken out than naturally goes into it.
“most of the agriculture west of about central Nebraska is dependent on irrigation”
See (my) post #60