If you use a 8Mhz 3v chip like in the arduino pro the current is very low. You can sleep the arduino and the xbee and quickly wake them up for transmission. I have a remote data logger running on 2 AA cells waking a few times a day and getting 15K transmissions on one set of batteries.
You can 'pin sleep' the xbee to turn it off. The current drain should be about .04ma sleeping, 4ma processing your data and 50ma transmitting. A cr123a 1300ma hr lithium camera battery should last you a season.
Sounds like a neat project.