I base my knowledge on experience. If I don't drink basically the same quantity of water to the amount of soda and energy drinks that I consume, then I'll stay thirsty. When I was smoking I would have to drink extra to account for that. I think the climate that you are in will affect how much fluid you need to drink too, i.e. when I'm working in Texas I have to drink more water than when I'm home in Lousiana.