I have kind of measured the difference, but it wasn't quantitatively accurate as you can't measure the amount of water lost via evaporation in an open waterer.
Well, now that I think about it there might have been a way to measure evaporation, but I didn't.
But I have always monitored the amount of water used each 24 hours by putting out the same amount in the open waters each day, as well as marks on the nipple water jugs to estimate how much is consumed in 24 hours....so pretty good comparative data, I think. I also kept in mind environmental variations such as temperature, rain puddle/snow availability, other moisture laden foods given, etc.
I concluded that they drank about the same amount of water out of the nipples, maybe a little less, as they did from the open waterer....but I didn't see any detrimental behavioral or health issues after going over to the horizontal nipples. They didn't gorge on puddles, chow down on snow, any more with the nipples than they did when they had an open waterer.
I do put out large shallow pans of ice water in the runs on very hot summer days when I see excessive panting, and they always go for that big time (some will even wade in them).
