Water mass fraction within the ice line (i.e. integrated over the region with a radius smaller or equal to the position of the water ice line) normalised by the initial value against time. The panels from top to bottom show the three disc viscosities. We note that for α = 1 × 10⁻³ the x-axis only spans to t = 5 Myr, but the late evolution does not deviate from zero which can also be seen in Fig. 2. The colours correspond to different simulation cases, and arrows indicate which planet causes the water vapour to increase by crossing the water ice line.