Water is pretty fungible, so while it might not be that alfalfa fields in the Arizona and southern California desert are irrigated from a residential pipe, it's the same water either way. Whether the water arrives at the field/datacenter via municipal pipe or irrigation canal doesn't seem relevant, except that a municipal pipe probably could supply most datacenters' needs, while desert alfalfa has a whole different scale of usage.

But if your point is that no one would farm alfalfa in the desert and use water that would otherwise be available for residential use, you are quite wrong.