Ab Initio embedded cluster calculations were performed to study water adsorption on Al-terminated (0001) alpha -Al2O3 surface. We used the surface charge representation of the embedding electrostatic potential (SCREEP) model to give an accurate representation of the Madelung potential at the adsorption site. The geometry of the cluster was optimized to take into account the surface relaxation. Adsorption energies were obtained using the N-layer integrated molecular orbital model (ONIOM). In the case of water adsorption it was found that both dissociative and molecular adsorption minima exist, with adsorption energies of -31.57 and -23.40 kcal/mol, respectively, in agreement with experiment. Bond orders, covalences and full atomic valences were analyzed to investigate the changes in the chemical bonding during adsorption. Our results provide some insight into the scrambling of water and crystal oxygen atoms during water desorption in isotopic exchange experiments.