Logarithmic model/observed intensity ratios for the H2O 1029–936 (321 GHz)/818–707 (63 μm) lines as a function of H2 and o-H2O densities. Contours are logarithmically spaced where the zero contour, if it were present, would correspond to the observed line ratio. The red region is where the 321 GHz line is masing. The temperature is set to 600 K, similar to that used by Carr & Najita (2011). The cross marks the Carr & Najita (2011) LTE solution.