Effect of collisional excitation on the H₂O fluxes of the submm lines 2−6 as a function of n_H2. The ordinates show the calculated line fluxes relative to the model that ignores collisional excitation. T_gas = 150 K is adopted in all models. a) In the case of moderate T_dust and low τ₁₀₀, collisional excitation has a strong impact on the H₂O fluxes at n_H2 of a few × 10⁴ cm^-3, especially on line 2. b) Collisional excitation is negligible for high τ₁₀₀ and very warm T_dust (note the difference in ordinate scales in a) and b)).