Top: population fraction as a function of radius for the ortho (solid) and para (dashed) levels contributing to the four detected lines for the best-fit model for IRAS4A. Bottom: cumulative contribution (I(r)) for the H₂O 1₁₀ −1₀₁ (solid red), 2₁₂ −1₀₁ (solid blue), 1₁₁ −0₀₀ (dashed red) and 2₀₂ −1₁₁ (dashed blue) lines for the best-fit model relative to the maximum absolute integrated intensity in each line (|I| _max).