[swaghauler]

Based on what I learned in my survival training (at ACE Adventure Center WV) and Cold Weather Mountain School (with the 10th Mountain), a soldier needs 1 LITER of water per 1000 Calories they expend per day. This volume of water (about 4L. to 5L. per day on average) is modified by the following situations:

Extreme Hard Work (digging foxholes, marching with a heavy load, etc...) X2
Cold Weather (32F to 0F) X1.25
Extreme Cold Weather (Below 0F) X1.5
Warm Weather, Humid (80F to 100F) X1.25
Warm Weather, Arid (80F to 100F) X1.5
Extreme Warm Weather, Humid (101F+) X1.5
Extreme Warm Weather, Arid (101F+) X2

Elevation above 5000ft adds X0.5 to the above climatic conditions.
Elevation above 10,000ft REQUIRES constant hydration (with the X0.5 add) to avoid HAPE (high altitude pulmonary edema) and HAPE can occur at lower elevations (as low as 8,000ft for the uninitiated). Hydrate or die in these situations.

All of these modifiers add together to create the water intake needed. For my Africa campaign, I just specify that the team needs 5L per day EACH. The native NPCs in my campaign need 4L per day. In a European campaign where the characters were marching, I'd say 3L per day would be appropriate for an average consumption. If you had a campaign in Iraq during the summer, 10L per man would not be unheard of if they were marching with weight during the heat of the day.

If the characters do not get enough water but at least half of the daily requirement, I allow a roll under CON on 1D10 to avoid any ill effects. A failure increases Fatigue by one Level (ie Light becomes Moderate). Drinking LESS than half of the daily requirement results in a 1 Level gain in Fatigue per Period until you hydrate.

I have posted a Water Contamination Table in the Water Purification Thread in the Forum Map. I hope this helps but my data is about 10 years old and the charts above involve worse case scenarios. I know the Army has a study on the internet about hydration as well.

Swag.