hm, how many times does a dwarf drink a year? 14 drinks of booze and 8 food, or 14 food and 8 drink? can't remember for the life of me. Something like that.
Of course... As you're not drinking booze they'll take longer breaks, but I'm not sure if that means they'd drink more. I think it will last you until it rains again at least. I'd get drilling to that cavern layer fast lol.
I was going to say before if you started in a scorching climate you'd probably have to tap the murky wells immediately on embark, guess I should have said it. Evaporation is insane.
"Hot" seems to be enough to create seasonal pools. I got 2 rains before summer hit, and have two pools "tapped" with a small ditch off to their side to collect runoff, and a well above each ditch. I'm halfway through my water reserves now though, but I think I've demonstrated to my satisfaction that it should be possible in theory to collect enough rainwater if I get my act together and dig collection ditches under other pools as well.
Building a roof over a pool will stop it from evaporating. It seems "inside" is sufficient. Building a bridge over a pool also makes it "inside" but I don't know if it can collect rain if the bridge is retracted. I suspect it can't. I also need to test the effect of floor-grate coverings, but it will be several months before the rains come again.
I have built walls around the pools at the surface to stop them from being doorways into my fort. Removing their ramps is not an option because (as I found out) it destroys the "murky pool" attribute that lets them collect rain in the first place.