I second the motion that a gallon a day per person is a minimum. A personal anecdote: I once walked across the Grand Canyon in a single long summer day - about 24 miles if I recall correctly, temperature at the bottom about 105 or so. I drank upwards of three gallons of water that day while hiking, and more afterwards. If I had had only one gallon I would have been in pretty bad shape by the end of the day. Fortunately I did not have to carry it all at once since there are water sources along the route.