Evolution of the rate at which the vapour masses of H₂O (left) and CO₂ (right) are changing in a smooth disc model in Scenarios 1 (purple), 2 (gold), and 3 (red) as a result of delivery by drifting dust and viscous advection onto the star. A typical rate for cosmic ray (CR) photodissociation of CO₂, 10⁻¹⁴ s⁻¹, is shown as the dashed lines as a reference for the rates at which chemical reactions occur.