Correlation between the optical depths of the ortho-H₂O 1_1, 0 − 1_0, 1 line and of the deconvolved profile of CH 3/2–1/2 (given for an equivalent hyperfine component with normalized intensity S​_ul = 1) along the SW (left) and NE (right) lines of sight, respectively. In both lines of sight, we assume a source-covering factor of f_c = 1. The colors encode the velocity intervals shown on the line profiles (top). The dashed lines correspond to opacity ratios τ(H₂O)/τ(CH^*) = 3 and 9, respectively.