You would have to maintain about 1-3 gallons per minute, depending on how cold the temp is, in order to not freeze the water. We get down to -30 deg. F. regularly (except this winter). Maybe if you're only getting down to +30 deg. F, it would be far less.

For rodents to drink that much, though, you're talking about LOTS of tubs. I don't know how many rats it takes to even get through 1 gallon per day, but with 2 gallons per minute, that's 2,880 gallons per day.

Quite a few tubs needed at that many gallons.