Lynne has a post on waterfootprinting, and it got me to thinking of the reason why footprints matter or should be used.
They should be used when we need to know the non-price impact of a behavior, e.g., our "carbon footprint" tells us how much harm we are doing to the environment. We do not worry about "coffee footprints" because we pay the full price of coffee when we buy it.*
Anyway, the whole idea of (or need for) water footprints stems from underpricing water, and water is MUCH easier to price for sustainability. First, because water supply and demand is LOCAL. Second, because water is a renewable resource that we merely reuse. (We are mining and emitting fossil carbon...)
Bottom Line: A water price that keeps demand in line with supply is sustainable, and a sustainable price does not require that we pay attention to any other measures of water use (i.e., footprints).
* Purists may say that we are not paying the price of indigenous land rights, seed biodiversity, the carbon from transport, etc., but let's leave those costs to separate footprints.