a) Proposed composite model for the H₂O submm lines in Arp 220 (see G-A12), compared with the observed line fluxes (black squares, from Rangwala et al. 2011). Toward the far-IR optically thick nuclear region (blue symbols), the E_upper< 400 K lines are expected mostly in absorption. The H₂O emission is generated around that nuclear region, in the C_extended (G-A12) component (green). Red is total. b) Resulting predicted composite PACS/SPIRE H₂O continuum-subtracted spectrum of Arp 220, which is dominated by absorption of the continuum.