We measured the pumpdown curves of an A5083 vacuum chamber and analyzed the outgassing in terms of desorption energies of water. The outgassing curves follow a ~$t^{-1.15}$ behavior before bakeout, which can be described by the first-order desorption of water molecules in the oxide layer. Analysis of the curves reveals that there exist several adsorption sites on the surface for water in the pressure range of ~$10^{-5};-;10^{ -8}$Torr. Measurements utilizing the throughput method show that the room temperature outgassing rate is ~1{ imes}10^{-13}$ Torr $ell$/s $ extrm{cm}^2$ after 24 - h bakeout at $100^{circ}C$.