Evolution of the computed local optical depths of two transitions as a function of the flow time of the neutral fluid in our reference models, the stationary C-type (left panel) and non-stationary CJ-type (right panel). The o-H₂O 1₁₀–1₀₁ (at 557 GHz) and p-H₂O 1₁₁–0₀₀ (at 1113 GHz) lines are considered, and their optical depths are shown in blue and red, respectively. The corresponding total optical depths are indicated in the same colour code. The neutral temperature profile shown in black. The shock parameters are specified in each panel.