I think you would have to have an idea in real life what each would be and then try to abstract it to a game rule.
For water.
You need around 1 gallon of water per day per person for temperate temps and a moderate level of work.
If it's hot, then you would need more water (2-3 gallons per day per person or more) and perhaps additional salt.
If it's really cold (low humidity levels), then you would need more water (2-3 gallons per day per person or more).
Water weighs around 8 lbs per gallon, so you need anywhere from 8-24 lbs of water per person per day.
For food.
You need around 3,200 calories per day per person for temperate temps and a moderate level of work.
If you are doing very strenuous work or it's very cold, then the calories may be as high as 6,400 per day or more.
If you are doing very little work, then the calories may be as low as 2,000 per day.
A pound of grain has around 1,600 calories, so you are looking at anywhere from 1.25-4 lbs of food per person per day.
Other foods usually have less calories per pound than grain unless they are high in fat, but then spoilage may be an issue.
So, for an expedition of a party of 6 adventurers for a weeks travel across an arctic desert with no known water/food sources, you would need to have around 1,008 lbs of water and 168 lbs of food. You would need some way to cook the food and boil the water, so you would need to carry some fuel source (you might be able to carry just fuel if there is snow/ice and melt it at each stop). The fuel source would need to be something that wouldn't have issues igniting at low temps.
If you have pack animals to carry the above water/food (not to mention other items like camping equipment, cooking equipment, etc), then you have to factor in additional water/food for them which may require additional pack animals to carry that water/food.
