This is badly framed question from a political perspective. California prides itself (or at least its farming communities do) for providing 1/4 of the (domestically produced, I assume) food for the US (although TBH I'm not sure how that is measured exactly), but WaPo is happy to cite the USGS on this, which says:
Using fewer than 1% of U.S. farmland, the Central Valley supplies 8% of U.S. agricultural output (by value) and produces 1/4 of the Nation's food, including 40% of the Nation's fruits, nuts, and other table foods.
Pride aside, when crops fail, farmers lose their income, and can lose their land. That's a lot more of a "voting emergency" than if someone's house value drops a bit due a dry lawn, e.g.
According to a UC Merced study conducted for the state, California farmers left nearly 400,000 acres of agricultural land unplanted last year due to a lack of water. The result, the study found, was a direct economic cost to farmers of $1.1 billion and the loss of nearly 9,000 agricultural jobs.
According to the WaPo article in many of those areas, the alternatives (right now) are... jobs in the prison system.
And yeah, some measures were taken that in the long will cut down agriculture in CA:
In 2014, the state legislature passed a law that requires water districts to eliminate any “overdraft” in pumping — removing ground water faster than it can be replenished — within two decades of its passage.
[...] a senior research scholar at Stanford’s Woods Institute for the Environment [said] “We’re going to end up fallowing millions of acres, so politically what does that look like? [...]”
Basically CA has decided not to farm more than it has water, i.e. no water imports in the long run (20 years). That's generally the kind of time frame you can do "structural adjustments" in agriculture (although in some cases such adjustments can be cut down to 5-10 years time frames).
Regarding "essential crops", it doesn't look like CA has such a def, leaving it to the market to sort it out in that regard, e.g.
Another crop that can thrive in extreme heat? Afalfa. [...]
Alfalfa is what dairy cows eat and California is big in dairy production. Dairy products and milk are valuable commodities, pulling in close to $7.5 billion per year but, in a drought, many people question whether crops like alfalfa should be grown here. [...]
[UC Riverside economist] Thornberg points out how 16 percent of the alfalfa grown in California is shipped all the way to Saudi Arabia to feed the dairy cattle in that desert realm. The irony? Saudi Arabia banned growing alfalfa because of water shortages there.
As for which farms get the water...
"In Western water law, you have 'first in use, first in right,'" [an alfalfa grower in Blythe] said.
Apparently some Saudi and UAE companies even bought land in CA based on this "first dibs" water rights laws (which seemingly transmit endlessly to new owners). 10% of those alfalfa exports back to their own countries apparently comes from land thus purchased in CA.
