  Circumstellar disks provide the material reservoir for the growth of young
stars and for planet formation. We combine a high-level radiative transfer
program with a thermal-chemical model of a typical T Tauri star disk to
investigate the diagnostic potential of the far-infrared lines of water for
probing disk structure. We discuss the observability of pure rotational H2O
lines with the Herschel Space Observatory, specifically the residual gas where
water is mainly frozen out. We find that measuring both the line profile of the
ground 110-101 ortho-H2O transition and the ratio of this line to the 312-303
and 221-212 line can provide information on the gas phase water between 5-100
AU, but not on the snow line which is expected to occur at smaller radii.
