In the US roughly one-third of all agriculture is irrigated from the Ogallala aquifer. We are currently using this water at a rate far higher than the aquifers replenishment rate. This means it will run dry. This means the water source will no longer be available. It has about 25 years left at current rates. We have temporarily exceeded the long term irrigation capability of the Midwest. Once you remove the aquifer as a source of irrigation water you remove that one-third of current agricultural output as well. It's a pretty easy concept to understand once you remove ideological blinders. Maybe you can find a cost effective alternate water source, maybe you can't. We are "spending" the planets "principal" and you can't do that forever. Once the principal is gone the lifestyle it sustained is no longer unsustainable.
John Thornton