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)?