i rode 110 miles on Monday, (9 1/2 hours) across hot, dry and dusty trails East of the Cascades and back to Seattle,
carrying over a gallon of water on my bike ( 4- 1 liter Zefal Magnums and an extra 1/2 liter) and drank them all, then refilled and finished most of those too (i think i had a quarter liter left) before hitting my doorstep, where i consumed another couple of liters.
that put me pretty darn close to 3 gallons for the day.
I drank about that much the day before too, on the 110 mile ride from Seattle up and over a mountain pass to eastern washington. out of the 220 miles total for the weekend, about 85 was on dirt, which seems to make me thirstier because of the greater perceived or actual effort...
If someone is only drinking a half gallon a day on a full day of riding loaded touring bikes in the summer, i would almost guarantee a small degree of dehydration and the associated reduction in your performance and comfort. these people must be fond of midafternoon headaches and the evening heaves, eh?