Oh no, the US will starve relying on the enormous strip of super fertile land between Texas and Idaho.

Point is, the US needs to consume the food more than we need to grow it. The water is not that great a threat.

Actually if California stopped producing food we would still be fine. Most of what is produced is fruits.

Most likely the US could halt 1/2 of its food production and still have plenty of food. We have no shortage of arable land, we have a greater shortage of people actually wanting to be farmers/ranchers :(

20% in terms of dollars, perhaps.

Not in terms of calories.

The rest of the US could live just fine without out-of-season artichokes or whatever.

Water is rather more important. You can live a long time on a plain diet (especially if you have vitamins available). Without the water, things in California would get very, very ugly, very, very quickly. ]

Also: what are you planning to do if (or maybe "when" would be better) Mexico decides that it wants to take back its former territory (i.e., all of California)?

Even in drought, California has a lot of water. Imported water is only necessary so long as we irrigate the central valley. We wouldn't need it just for residential use.

I think an independent California would have a good relationship with Mexico. Though I'm interested in why you think we wouldn't.

Desirable land, no longer defended by the US military, and a fair number of their citizens here. I'm not sure I agree that it'd happen, but I can see some reasons.