entire regions being deserted is not a new thing (see mayan civilization). it's possible we'll see the cities of southern California (San Diego, Los Angeles) as remnants of an ancient culture in a few hundred years, left intact by the scorching heat and desert.
San Diego seems to be doing pretty well at water management. The naysayers weren't able to block the desalination plant and they are doing other things too.
They use lots of energy. And then you need to get rid of hills of salt. Stuff that obviously can't be just put in any fertile soil or returned to the sea near the coast. A partial solution could be to use it in winter roads, but then you need to burn gas to move it to the North. At some time, you will cross the line where the water obtained is cheaper than the energy used to produce it plus collateral effects. And at this point the plant is not profitable and is just burning money to run.
So, I hate to say that, but there is no magic solution at sight, still.
This planet has a fabulous technology available for making excellent pure water. The problem is that we burn them all, each year.