Modeled L_H2O/L_IR × (100 km s^-1/ ΔV) values for lines 2 (upper), 4 (middle), and 6 (lower) as a function of τ₁₀₀. Collisional excitation is ignored except for the red dashed lines, where T_gas = 150 K and n_H2 = 3 × 10⁵ cm^-3 are adopted. The small numbers on the right side of the upper panel indicate the value of T_dust in K, and those on the left side indicate N_H2O/ΔV in cm^-2/(km s^-1). When compared with observations, the modeled L_IR values should be considered a fraction of the observed IR luminosities (Sect. 4.3.1). For τ₁₀₀ = 12 and T_dust = 95 K, line 4 is predicted to be in absorption. For fixed T_dust and τ₁₀₀ ~ 1, the L_H2O/L_IR ratios are similar for very different N_H2O, indicating line saturation.