While water is renewable, there is a limited supply of cheap water for crops, and some areas are using it at a higher rate than it is being replenished. Wikipe

While water is renewable, there is a limited supply of cheap water for crops, and some areas are using it at a higher rate than it is being replenished. Wikipedia mentions that the Ogallala Aquifer, which is used to irrigate a significant portion of Midwest farmland, may run dry in 25 years: http://en.wikipedia.org/wiki/Ogallala_Aquifer http://en.wikipedia.org/wiki/Ogallala_Aquifer For the people in that area, the aquifer is their "local dam" and it is getting low.