One assumption that I think some folks make is that as the world population increases, there will be an intensification of the effort to produce food.
But more people, means more people wanting water, and urban folk usually "out vote" rural interests, even if the long term interests of the rural folk would (somewhat) be beneficial to the urban folk.
Michael Hiltzik, Los Angeles Times, 20 June 2014 (hat tip: NC)
Another assumption being challenged is the primacy of agriculture's claim on water. The solution is to buy farmers out, trading cash for their water rights to keep supplies flowing to urban areas. The MWD is working to develop a plan to pay growers to fallow their land to raise the water level of Lake Mead. "But we really don't know what the response will be to a cash offer to take land out of production," Kightlinger says.
The reason I say "somewhat" beneficial, is that in a global market, the water used by agriculture will not necessarily go toward local use.