Top: rotational diagram of H₂O calculated assuming a kinetic temperature T = 1000 K, H₂ density n = 10⁶ cm^-3, H₂O column density 2 × 10¹⁶ cm^-2, and line width ΔV = 5 km s^-1. Lines observed in absorption in NGC 6334-I are shown in blue. Fits are done to all lines in the PACS range included in the LAMDA database (Schöier et al. 2005) (in black) and to the NGC 6334-I lines (in blue). Darker shades denote optically thin lines. Bottom: the same as above, but assuming an H₂O column density 10¹² cm^-2, such that all lines are optically thin.