This is a totally unscientific observation, but I did a run yesterday: http://www.sportstracklive.com/track...ath/Run/129894 according to the weather stats on it, it was 8 degrees and 93% humidity, it was raining at the beginning but had stopped by halfway. I was sweating like a pig, as I always do! I drank about half a litre of water. I probably drank more than I needed to ( I also ate a cerial bar at the half way mark) as I want to get into a rythem of eating and drinking as I up my mileage, but I am sure I would not have been very comfortable if I had not drunk anything.
When I was riding long distances Polaris Challenge etc I found to stay comfortable I had to drink about a litre an hour to stay comfortable. I am guessing I will need about the same when running.
James