[smeyer]

I don't think the metaphors would be evaded by using metric units, though. Would 4 Teraliters been more useful to much of anyone?

Converting it to gallons, liters, or any other common unit of volume probably leaves most people without a sense of relevant scale unless you use a metaphor (e.g. how many households it could sustain for a year) or otherwise tell them how much water the states use (e.g. including a percentage of total usage).