H₂O/H₂O line-strength ratio versus the H₂O/CO line-strength ratio for a selection of H₂O transitions. The points with error bars give the measured line-strength ratios, color coded according to the Ṁ_g/ ν_∞,g range to which the sources belong (in units of M_⊙ yr^-1 km^-1 s), as indicated in the first panel. Undetected lines are not included in the figures. A black cross superimposed on the data point indicates that one of the H₂O lines is flagged as a blend. The colored curves show the predicted line-strength ratios for various values of Ṁ_g/ ν_∞,g, in the same range as the data points for each color. Each row of two panels shows results for different H₂O abundance profiles (see Sect. 5.2.2 for more details.) The crosses superimposed on the curves indicate the models for an increasing H₂O abundance from left to right in factors of 10, with the highest maximum value being 10^-4. Adopted values for other parameters are listed in Table 4.