Except coffee is a diuretic, and your water intake will be more than depleted by the peeing you do because of that effect. If you drink a lot of coffee during the day at work, and wonder why you look old and haggard in the face every morning, and you feel lethargic, and you visit the restroom often, you are likely dehydrated as a matter of course.

Oh yeah, finished the first century just now, and we remained reasonably hydrated... and fed... and electrolyted thanks to good old Enduralytes.

HawkOwl also is right about that cool weather thing, and if anyone reads the Long Distance forum thread, they will see a reference there that there is as much danger of dehydration on a cool,. intense ride, as there is on a moderately warm ride at the same ambient temps.