Interesting. How are you going to measure your water loss? Based on how much you drink?

Personally, I've tried doing this experiment a number of times and I just weigh myself before and after some activity. The difference in weight, minus an adjustment for water I drink along the way, gives me a decent idea of how much I lost and how much I might still be short, even after drinking.

One thing about cold weather activity that irks me and might skew your experiment (at least it does for me) is thermal diuresis--i.e. the diuretic effect of the cold. Within 20 minutes of going out into the cold from a warm place, I suddenly have a really full bladder that I need to pee away. It's not THAT critical, but you will overestimate your water loss during some activity if you weigh yourself before the thermal diuresis kicks in when you first go outside. I mean, you really are losing that water, but it's not actually activity-related water loss.