Points well taken. I'm not there, so don't know how creative you want
to get. I am on a well (which, no, is not exposed to the atmosphere --
it is sealed, as should all wells be -- although if it got completely
submerged by a flood I would probably want to have it tested just in
case it leaked). Um, sorry...

I am on a well, and have a standard pressure tank that holds maybe 40
gallons or so of water max, and keeps at minimum about 20 gallons in
the tank at all times. When it gets down to around 20, the pressure
gets too low, which trips a switch and turns on the well pump. When
the power goes out (taking our well pump with it), we have somewhere
between 20 and 40 gallons of water left in the system. The last 10 or
so would be at very low pressure -- but can be retrieved in the
basement pressure tank drain if needed. We get our drinking water at a
spring, using cleaned gallon milk jugs. We get a few days worth at a
time (8 gallons or so) -- after 2 weeks or so the water would get a bit
funny the way we store it. Even so, we still have to discard the jugs
after a while and replace with new ones.

You might do just fine with just a well pressure tank (or two), fitted
in some simple manner to your city water supply. I imagine something
like this might work:
Use one, two, or more pressure tanks, hooked up using tees. Use just a
single pressure switch, located one one of the tanks. Hook the pressure
switch up to an electronically-controlled valve that opens the city
water connection. Normally you would be using water from the pressure
tanks. When volume (and pressure) gets too low, the pressure switch
would open the city water valve, replenishing up to full volume (and
pressure).

-Kevin