The adsorption of water monomers, small water clusters, and water thin films on $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ surfaces is studied by density-functional theory. For the metal-terminated surface, the calculations favor the dissociative adsorption for low coverages and the formation of hexagons of alternating dissociatively and molecularly adsorbed water monomers for water-rich conditions. The calculated adsorption energy per water molecule decreases from about 1.5 eV for single adsorbed molecules to about 1.2 eV for thin films in very good agreement with our temperature programmed desorption experiments. The fully hydroxylated (gibbsitelike) surface, however, represents the thermodynamic ground state of the $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ surface in the presence of water.