Given the lack of services on our route through Nevada and Utah, access to water is a huge concern for us. There are some long stretches with minimal to no access to water. I remember running out of water for a few hours on a bike tour when I was in high school on a hot day and it is not a pleasant memory. I have 3 water bottle cages on my bicycle and I can carry 3 bottle with a total volume of 2.7 liters of water. Water is very essential, but very heavy. One liter of water weighs about 2.2 pounds. So 10 liters of water which is about 2.6 gallons weighs 22 pounds!
After much debate, we decided to bring collapsible water bags from MSR. I am bringing two 2 L DromLite bags. I plan to carry one in each rear pannier when I need extra water. I wanted to keep the water as low as possible on my bike to improve handling. This means when carrying extra water I probably will need to store some gear on top of my rear rack. I plan to use an extra stuff sack to hold the gear.
John only has room for 2 water bottles due to fenders on his bike so he opted for the 10 L Dromedary bag from MSR. He has a sturdier rear rack on his bike and he plans to carry extra water on top of the rack. The Dromedary bag is more rugged than the Dromlite. I think the Dromedary bag is a good choice especially since John will be touring for 2 months. I also think the bags will be very useful when camping to supplement the water bottles.