How much water do you need each day? Boy, that's a loaded question. It depends on so many things, from activity level, to temperature, physical condition, food intake, etc....

I typically carry two 1-liter Nalgeen bottles with me in my pack when I'm hiking, assuming we'll have access to a place to filter water and refill them regularly. If we're not going to be near a place where we can get water, the equations change obviously.

I took a look in the "AMC Guide to Outdoor Leadership", and the author, Alex Kosseff, makes that 2 liter minimum per day recommendation, but, also goes on to say: "Two liters a day is a bare minimum - I have led groups where the 5 liters a day (at almost two pounds per liter) we carried was insufficient in the hot and dry desert."

I would note that in cold conditions, dehydration can really sneak up on you. In the desert, you know you're loosing water through sweat. In cold dry conditions, you're loosing a lot of water two, but, you're less conscious of it. You're actually breathing in very dry air and exhaling really moist air.