Facility location models usually take a fixed time scale to determine customer demand, e.g. the annual demand. In some situations demand is seasonal or is expected to change in the next coming years. The models then typically use multiple periods in which the demand is specified. One can then also take into consideration how the other data (costs, capacities, available sites) will change from period to period. These models then may consider only using a facility over one or a few of the total number of periods considered; the facilities used in each period as well as the flows through the network may then vary from period to period