Im not sure how much water they drink. But i know they get it not only from the water we put in our pens but also in there diet. Plant life and the digestion process itself. I do know from talking to some of the Urine guys in our industry that they can collect up to a gallon of urine from one doe in one night. So i would think it would be a considerable amount more then what they are passing.
I know for us here that one of our pens went through 5 gallons a day during the winter. ( when water intake is suppose to go down) between 3 deer. Another pen would do 5 gallons every two days between 4 of them. So i just think it has a lot to do with size, condition, and diet in your animals. You know whats funny about it all is that you can have a freshly filled pail in there and they will still choose to drink out of a puddle.LOL But We try to keep it fresh and clean as often as possible.
I would be interested to see if someone has actually measured the amount of water it would be nice bit of info to know.