Diagram of the model considered for the relation between the ChEH, the SFH and the gas accretion history. The change in the metallicity of the gas over a time interval (t₁ to t₂) is the result of the metal input from the stellar populations created by the SFH up to that point and the dilution due to the accreted pristine gas. The model does not consider other mechanisms such as outflows or mergers whose effects of dilution or over-enrichment will therefore be conflated with the balance of gas accretion.