Coming late to this one but just thought I'd add a couple of things - firstly when hiking/camping around 15 miles a day this summer (not especially hot but still...) I only went through 1-2 litres a day, not including washing.
Also the often cited '2 litres a day for health' notion is a myth,
http://www.theguardian.com/lifeandstyle ... -water-day
But also remember that a lot of our water comes from food and if you are relying on a lot of dried food then that will ofc affect things.
So although it would be nice to allow 3-5 litres a day per person I certainly think you could manage on a bit less, especially when 'bugging in' and not being unusually active.
A typical water butt contains 150 litres so for a family of 5, even allowing just 2-3 litres per day each, that is 12 days supply (per water butt) without doing anything else to obtain water and assuming no rain falls.