There is no "return to the norm" associated with "drought." Mirriam-Webster and Wikipedia agree on "prolonged", MW adds "indeterminate." Jared Diamond argued in Collapse that drought is the number one killer of civilizations and that generally you see flourish followed by irrigation followed by silting followed by abandonment.

I'll say this - I grew up in New Mexico and could not for the life of me fathom how the fucking Anasazi did it. They clearly set up in a terrible place with nothing to eat and nothing to drink. It wasn't until high school that anthropologists started talking about "well yeah when the Anasazi were here it looked like South Colorado not East Arizona". All of them agreed that "drought" broke the Anasazi, and all of them agreed that it took decades. Hell, if you studied what was left of their structures (the Pueblo tribes) you discovered that each and every one of them was defeated by the Spaniards when they laid siege and kept them from their water.

Dust Bowl? yeah that was caused by the US thinking a ten year wet period was normal, and the Dust Bowl was called a drought. California for its part exists because California dammed every river they could find, tapped every aquifer they could reach and went after the water like it was gold. That's not new either, it's the plot of Chinatown.

The place can't naturally support 40 million people. There's no way around that. The Daily Shot had a graphic I can't find right now where they cross-referenced "places people are moving" with "places that are running out of water" and the correlation was nuts. We've had a bunch of clients bail for Arizona because the cost of living is lower and it's like... don't get used to it?