Have you done a test ride of your route, on a weekend morning, for instance? How long does it take you? (Guessing about an hour...)
I'd suggest planning on drinking a quart of water per hour of riding, which probably means two water bottles for a one hour commute. Plan on dousing your jersey and the back of your head and neck with some of the leftover halfway through.
If there are thunderstorms in the area, the bus is a great idea. Spectacular rainbows over Tucson when that happens!