I'm talking about the entire southwest, not just San Joaquin Valley.
In agriculture, Riverside County is the 13th most productive county in the state in planted crops with $1.2 billion in cash receipts on about 210,000 acres [1]. That's more than any single county in Florida [2] based on crop value and the entire state of Florida produces 5-6 times more agricultural products than Riverside County on ten times the acreage. Most of that is in the Coachella Valley that is adjacent to Palm Springs/Desert Hot Springs and this productivity is despite the massive environmental disaster that is the Salton Sea [3].
I don't have an agricultural report on hand for Maricopa County (Phoenix) but based off a cursory search it produces about a third of the agricultural output of Arizona, which as a whole totals half the cash receipts of Florida's industry, despite a third of the population (though I believe AZ is a lot less productive per acre than FL because it's a lot of pasture).
Now, don't get me wrong: analyzing agricultural productivity - let alone the sustainability of entire metropolises - in dollars per anything is a fool's errand that obliterates all nuance. I'm just trying to illustrate that most [4] of these cities weren't dropped in the middle of desolate wastelands by hopeless fools that kept on attracting more fools. They were founded because the deserts of the southwest provide an ideal environment and all we needed to do was bring a single ingredient. Just take a look at Phoenix in 1885 [5]: farmland as far as the eye can see, half a century before pumping mass amounts of groundwater for agriculture was even an option.
When it comes to the food people want to eat, each [4] of these regions more than pull their own weight [6]. Unless we plan on transitioning the entire country to a wheat/corn/beef/pork/chicken-only diet or significantly decreasing food security or quality of life for everyone, we have to come to terms with the fact that the entire country is not sustainable without the Southwest, which means the region's water supply a national concern. We could completely depopulate all the big cities in Arizona, Nevada, and California without saving as much as a fifth of the water we currently use because the rest is used to feed the nation.
[1] 2020 numbers: http://www.rivcoawm.org/Portals/0/PDF/2020-Crop%20Report.pdf
[2] Florida was first wet state that came to mind and it's top third to middle of the pack nationally in agriculture depending on the specific food group so I think it's representative. 2017 numbers: https://www.fdacs.gov/Agriculture-Industry/Florida-Agricultu...
[3] More precisely, farmers caused that disaster to begin with, but the area is recovering and they are are adopting more sustainable techniques.
[4] I'm ignoring Vegas because it's an outlier that started out as a supply stop for the United Pacific railroad. It only grew to its current ridiculous size when Nevada legalized gambling and the economic activity from the construction of the Hoover dam let the gambling industry develop a permanent foothold that sustained the city.
[5] https://en.wikipedia.org/wiki/History_of_Phoenix,_Arizona#/m...
[6] We'd actually need to look at export numbers to get a more accurate picture but i.e. nearly half of Arizona's agriculture is exported if I'm understanding the ag department correctly.
