The majority of California is desert, always had been and always will be. Yes, there is large aquifers there, for now. But the water is being pumped out far faster than it can be replenished. In many areas people have had to drill their wells deeper because the aquifers are dropping and their wells have gone dry.
The Sierra mountains get a lot of snow, most years, which provide most of the water for the rivers and aquifers. Some years they don't get enough snow runoff to fill the lakes, streams and rivers. And yet the farmers still demand "their" share. The Indian tribes still demand "their" share for the salmon runs, and of course the big cities still demand their share.
Generally I agree that government is the problem, but I also know farmers and ranchers very well. They would drain the state dry, if they were allowed to, and then move on. They've done it in many areas around the west.
Bottom line is there are too many people chasing limited resources.