Observed (blue) and modelled (grey) column-density-averaged water ice abundance ratios in the water freeze-out zone. The displayed models show the water ice abundances for pre-collapse times of 0.01, 0.1, and 1.0 Myr, followed by a post-collapse time of 1.0 Myr. Another run with t_post = 0.1 Myr (not displayed here) reveals a marginal dependence of the abundances on the post-collapse time, with the results being the same as for t_post = 1.0 Myr within ≲ 10−15 ppm.