In that case, one should add the water consumption of growing the alfalfa to that of growing the cash crop directly, and similarly add whatever value (monetarily or otherwise) the alfalfa has to that of the cash crop - in other words, consider them together as a unit of production.

I have no idea whether this results in a currently-justifiable practice, but it is a simple fact that change is going to be forced on agriculture (and all of us, as its dependents), one way or another.

Citation?

This seems to directly contradict your claim: https://www.theguardian.com/sustainable-business/2016/mar/08...

I know lots of people who grow only alfalfa out here. Some are family members. They sell it as feed, or rent out a field.

Small farms. Less than 600 acres. Not much water, here.

My grandpa was from a similar area (the Tulelake basin), and before they had trouble with water rights, alfalfa was exclusively a rotation crop, but you are correct that when water becomes scarce, that it falls into a category of the only thing that can grow reliably. California is not a truly water scarce place kind of place--water is just political these days.

That's not the case at all. Some fields remain alfalfa for long periods of time and it's a very lucrative crop.

I don't know what regions you're referring to, but my family gets $75/acre rent on their land when their lessees plant alfalfa and $300/arce when they plant potatoes. In the Central Valley, they can grow even better crops than potatoes.

S.W. US (Arizona and New Mexico).

Field remain alfalfa for many years at a time. More cuttings then in other parts of the country also.