Effelsberg (in black, spectral resolution δv of 0.1 kms^-1) and e-Merlin (in blue, δv = 0.1 kms^-1, the apparent absorption near –63.5 kms^-1 is attributed to e-MERLIN residual sidelobes, see Sect. 3.3) spectra of the 6₁₆−5₂₃ transition of ortho-H₂O at 22 GHz toward NGC 7538-IRS1 plotted over the SOFIA (δv = 1.1 kms^-1) spectrum of the o-H₂O 8_2,7−7_3,4 line at 1296.41106 GHz (in red).