Nerodia erythrogaster
Two subspecies of N erythrogaster are found in Alabama
N.Erythrogaster flavigaster (Yellow-bellied Watersnake)
N. erythrogaster erythrogaster (Red-bellied Watersnake)
This specimen is likely N. e. flavigaster as it is found throughout the state.
Reference: http://www.outdooralabama.com/watchable-wildlife/Reptiles/Snakes/pbws.cfm
Auburn, Alabama