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 (monetaril

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.