I second the water and duplicate filter comments. Mouse, as usual, is dead on in both cases. Here in the east, I normally carry 1 liter, which is just about right most of the time (sometimes, but not often, I'll arrive at a source with more than a few sips left.)

However, I do carry spare capacity in the form of the 4-liter "dirty water" bag that comes with my Platy Gravityworks filter. If I'm going to make a "dry" camp, I'll use it to carry extra water to the campsite from the last source before camp (rarely more than a mile.)