It generally takes just a day or two to completely drain water towers, and in a REAL disaster, all it takes is a few broken pipes to make that happen even in the absence of normal use.
So the timeline is power loss -> towers stop refilling -> towers empty, which only takes a day or two at the outside. The DDA scenario takes place several days after all that happened, so any water towers/resivoirs are assumed to be drained.
There is all kinds of things you could do with this though, like installing a huge water catcher on top of a water tower and using it as a massive water source.
I wasn’t aware of the pressure tank thing, that certainly seems like a valuable resource you could find in out-of-town homes. The engine doesn’t have support for such a thing, but it’s certainly doable, and they could spawn with a variable amount of contents (based on being used after a power loss etc).
It does bring up another issue witch is wells in general, which we could also support, both as something you find and something you build.
We don’t have any support for liquid physics, and we don’t particularly need it, we don’t support medium scale terraforming like minecraft or df, which is where that kind of thing becomes important.
We can re-use the vehicle electricity system to distribute pressurized water, it’s pretty straightforward, so there’s no reason to do anything hacky like store water in sinks.
