This might work in the UK. There is no national water infrastructure, so although it rains a lot, there are regular droughts - especially in the South East where most of the population lives.
There's been some noise about fixing this, but after privatisation there's no commercial interest in taking on national-scale projects without government subsidy.
Somewhere like CA is largely desert anyway, and there's no obvious wet climate area to collect water from.
So practically, only draconian conservation laws and perhaps a huge string of desal plants can save CA. I guess neither of those are likely. And then you get a repeat of what's happening in Sao Paulo and Rio in Brazil, where there's no water for days on end.
The terrifying thing about climate change is that it's literally making some areas uninhabitable. Large parts of FL have less than fifty years, CA and NM are drying out, the East Coast will become more prone to flooding and perhaps also to extreme winters.
Europe is going to have similar problems as flooding becomes common. There are already places in the UK where buildings insurance is no longer affordable.
At some point the economics stop making sense. Not long after that people either leave or become homeless.
It would have been smart to avoid these outcomes, but that doesn't seem to have happened.