Ok so I've had many Stress-Vomit dupes and open body of water below my base, here is what happened:
Before any of them started vomiting out of stress everything was honky-dory - pump would pump water at it's usual pace to water-using machines, get soiled and go back to water purifier, in turn it would return water to same utilities and dump overflow back to the pond. It drained some water, but pace of water drainage was minuscule.
Until dupes started stressing out and vomiting in the water body....so my initial reaction was: Nice, free contaminated water to refine and re-use in plumbing! but nooo.... vomit does not combine in a big pond, it seems like once dupes vomit, their vomit replaces liquid that was on floor already
and fresh water gets consumed in that process (if in contact)
vomit, it seems, "sucks up" fresh water in contact - so after couple day cycles all of my water in pond was magically GONE. I really doubt it is an intended mechanic, or maybe I'm not getting something?
open body of water below base > stress out vomiting duplicates > vomit replaces body of water after 3-4 vomit dumps (depending on body of water size)