They have been taking water from somewhere for a long time, cant they just take more of someone else' water in order to live in a desert?
Hey, the USA is a large and sparsely populated country.... How about you try living in some of the more habitable areas?
Nobody lives in the California Desert. Well, okay, we do have a decent retirement community out in Palm Springs, but the parts that most people settled on were temperate grasslands, forests, and wetlands (the Central Valley was an inland sea for much of the year before we dammed it all up).
The real problems are:
1) Irresponsible farming by agribusinesses. This one here is the biggie, but is really hard to control because the biggest agribusinesses have so much political clout, both here and in Washington.
2) 150 years of politics. For well over a century, the saying has gone, "Liquor is for drinking; water is for fighting." There are a byzantine set of local, regional, statewide, interstate, and international laws governing how water is used everywhere in the state, most of it based on environmental studies decades or centuries out of date, and none of it changes quickly.
3) Wetland destruction. For a long, long time nobody understood the value of wetlands in water table control, flood prevention, and ecosystem management, and so much of it was filled in and paved over in the last 100 years. This has proven to be a huge mistake, one that will take decades and billions of dollars to fix, and isn't helped by ignorant jackasses who insist that environmental concerns don't exist, that scientists are hucksters, and that God will provide everything we could ever want, forever.
4) Climate change. The theory is nearly 200 years old; the lab-scale proof is over 150 years old; definitive proof it's happening out in the environment is over 50 years old. It's happening, right now, and given politics and the endless prattle of ignorant jackasses it doesn't look like it's going to be slowing down any time soon.
Did you notice what's not on that list? Cities. All of the urban and suburban development in California accounts for less than 10% of the state's annual water usage (the vast, vast majority is used for agriculture), and the number is dropping every year, as more efficiency and water recycling programs come online.