I suspect that when water shortage really begin to crimp places like LA, Tucson, and Las Vegas, you will see conditions developing akin to those we see in the Rust Belt cities of the northest, most notably Detroit. In this scenario, there will be a gradual decline in population, not a catastrophic, Mad Max rush for the exits.

One of the advantages of Detroit is that the area possesses abundant water. So if you are really thinking ahead.....