One can't do *science* fiction without some occasional references to
technology; don't let ST poison the well here.
There's a fair amount of water on B5, all of it recycled carefully.
You have to allocate some drip-irrigation for stuff like the hedge maze
and a few other things because the mind needs stuff like that. I would
think that they manage to siphon off any growing humidity inside the
central area rather than allow for unplanned rainstorms that might muck
things up.
jms