Electricity Carries Water With It
Energy demand and water demand are physically coupled. Changing the power mix changes the water footprint even if the cooling system never changes.
The coupling
Thermoelectric power plants — coal, gas, nuclear — generate by boiling water and must reject waste heat, typically through cooling towers or once-through cooling. Every kilowatt-hour purchased from that generation carries an embedded water consumption. Wind and solar photovoltaic generation consume essentially no water in operation.
Consequence for data centers
A closed-loop, zero-evaporation facility drawing power from a thermoelectric grid still has an indirect water footprint. Conversely, an evaporative site on a heavily renewable grid may have a smaller total footprint than its onsite figure implies. Onsite and total are different accounts.
Open question
Where a facility reports water savings from closed-loop cooling, how much of that saving is offset by higher electricity draw for mechanical cooling, and by the water embedded in that additional electricity? This requires the site's power mix and is rarely published together with the WUE figure.