Energy level diagram of H₂O, showing the relevant H₂O lines at submillimeter wavelengths with blue arrows, and the far-IR H₂O pumping (absorption) lines with dashed-magenta arrows. The lines are numbered as listed in Table 1. The o-H₂O 3₁₂−2₂₁ transition is not considered due to blending with CO (10-9) (G-A10), and the far-IR 2₁₂−1₀₁ transition at 179.5 μm discussed in the text is also indicated in green for completeness.