I taken to touring with 5 water bottle cages and Zefal 1 liter bottles. Three cages on the frame and two mounted on the front rack, behind the panniers. I add a MSR Dromedary bag (usually on top of the back rack when full) I've been using a 3 or 4 liter dromedary bag for solo trips. I'm a water pig, and tend to use a lot making tea and just drinking it. They make a 10 liter bag, which is nice for two people but weighs 20 pounds when full!

For a Grand Canyon ride, across highway 50 and around the Great Basin type of desert touring, I'll have at least 2 1/2 gallons on the bike, if you are unsure as to the next water stop.

Usually, on most tours, around midafternoon I look for a good water source, fill up all my bottles and the drom bag for the nights cooking and making breakie. It's nice to start off the day with full bottles.