Velocity shifts (a–b), line widths (c–d), and equivalent widths (e–f) of the H₂O lines in NGC 4418 (left) and Arp 220 (right). Panels g–h show the line fluxes normalized to that of the 3₃₀ → 2₂₁ transition at 66.4 μm (see Sect. 2.3 for details). Error bars are 1-σ uncertainties from Gaussian fits to the lines.