Two-component, nonlocal radiative transfer models adapted to the physical conditions in the Horsehead. The upper panel shows the o-H₂O 557 GHz synthetic line profiles (continuous curves) and observed line profiles (histograms) at the emission peak. The lower panel shows the predicted excitation temperatures of the 1₁₀–1₀₁ transition. The blue curves represent the model of the dense PDR component alone. The red curves represent the model of the low-density envelope alone, while the black curves represent the two-component model.