Comment by eventualcomp

There is also buffer for clean water required - Y gallons on-hand at the datacenter. You can see the other replies in this parent comment demonstrating the tight ongoing humidity requirements, and how clean water is sprayed onto the actual hardware to cool it off, and more. Evidently this can't be done by setting up a giant funnel above the datacenter to collect rainwater.

Given those considerations I expect Y to be pretty large.