Earth's total water carrying capacity has remained more or less the same since the early days of the planet. Water never really goes away. It just goes through chemical reactions - back and forth between states. Beer in...pee out. Evaporate. Condense. Liquid water. Beer again. The cycle repeats.

Drinkable fresh water is another story. Desalination plants are in their technological "infancy" and are still much too expensive to be market stable and competitive. Besides massive costs, the next problem is what to do with all the leftover salt. You can't put it back into the ocean or you increase the overall salinity & kill PH levels. There are mountains of salt stacked up in the desert out on the Baja peninsula from Mexican desalination plants. That in turn, kills the ground, flora and fauna all around it permanently. The actual water tastes like crap too (think Florida tap water nasty).

The good news is that we are probably headed back into a mini ice-age. On-ground rainfall and snowpack is going WAY up in certain areas. Alaska has seen more snow in the last 200 years than since the days of the last mini ice-age (1300 - 1500 AD give or take). Of course, some areas are the opposite. The Sahara is getting drier and bigger by the day. It'll eventually push the entire north African population straight into the Med.

So where in North America will have good climate, water, farming and minimal demand in the future? The Montana & southern Alberta zone. Montana now has a climate like Wyoming used to have and Colorado had before that. Colorado is the new New Mexico. Drier than hell and it never used to be that way. The food belt is moving north. But as we all know, not too far north because the climate is not conducive to growing crops (unless the Earth axis tilts bigtime and then we wouldn't care anyway). Eventually there be only a very small band of optimal growing area running along the 45th parallel. Everywhere else is gonna be chit for humans. But in the end, none of this matters because it will take geological ages to occur. We won't be around for that long. We have like 200 years to get off the planet or that will be that. I for one think we'll tear ourselves apart within the next 200. It's gonna be a show!