Somehow the water used by [almond and whatnot] farmers is a problem in CA.

Rain is great, and Earth's water cycle is closed, but plants evaporate water but you won't get it back as rain immediately.

Plus if you install irrigation you are taking water away from somewhere that would likely not evaporate that fast. (Eg from a river.) And if you also happen to have drainage, (eg if you converted a wetland area), then you suddenly discharge too much water from that region, and you won't get it back as rain.

Most crops aren't irrigated.

I came here to dispute this, but when I looked it up, it appears to be true, at least in the US, where irrigated farms produce only 39% of farm products (in $). That being said, its still a nontrivial percentage, and irrigation for agriculture accounts for 80% of the nations water consumption.

https://www.ers.usda.gov/amber-waves/2017/june/understanding...

The disparity would be much larger per calorie rather than per $. Staple foods tend not to be irrigated.

Sure, but with the already significant climate changes yields are affected. Some regions turn toward irrigation, and it'll eventually run out, and it already affects non-agriculture water usage in many regions. (In India. But China's megacities also consume more water than what the natural replacement rate of the regional water table.)

I'm not saying everyone has to go all in on vertical farming, but there's value in food safety.