[The Dark]

This seems pretty good. I've read research that water consumption should be around 1 milliliter per kilocalorie, which is the same as 1 liter per 1000 calories. However, people eating wild or civilized food (or modern MREs that don't rely on freeze-drying) should be able to get about 20% of normal water needs from the food they're eating, so about half a liter to a liter of water can be gained from a day's rations.

For exertion, I think I'd tie it in to the four-hour periods, so that each period of Easy Work adds x0.125 and each period of Hard Work adds x0.25, starting from a typical base of 4L (based on 3 MREs being 1.5 kilograms and ~3750 kilocalories) AND apply the amount after accounting for weather/altitude (i.e. in Hot, Arid weather, the x0.25 applies to the doubled base, rather than being added to the x2 multiplier).

This would mean that in Europe, each four-hour period spent marching would require an extra 1L of water that day, while time spent foraging would require an extra 0.5L per period. Marching for 12 hours would require a total of 7L of water (4L base and 3L for three periods of hard work). In Iraq, the base would be 8L, and you'd need an extra 2L per period spent marching. The exact numbers may need tweaking, but safe water should be a concern.