Land and surface water levels decrease during long, dry periods. With this decrease the chance of desiccation, salinisation and ground level decline increases. In order to counter these effects, fresh water is supplied in the water-level controlled areas. This fresh water can come from other surface water bodies, such as rivers and the IJsselmeer, deeper groundwater and the sweet effluent from water treatment plants. If there is a shortage of fresh water, the displacement series will take effect. The displacement series determines which user functions have priority over others in the supply of fresh water.