I have no scientific backing to this statement, but I don't feel I sweat even close to as much when it's cold as when it's warm. I hardly have to drink anything during rides in the winter.
As long as you don't bundle up so much that you're overheating, I don't think you need as much water in winter as during the summer. Sweat is produced for temperature regulation, so if you're cool/cold, you won't sweat as much as if you're hot.
edit: after researching several sites, they all said that you sweat less when it's cold, but the only metric I found was you sweat 1.7 times more during summer than winter.
Last edited by branman1986; 10-23-06 at 04:50 AM.