The importance of temporal scale when optimising abstraction volumes for sustainable aquifer exploitation: A case study in semi-arid South Portugal

Aquifer sustainable yields are often defined as a single value based on long-term averages or annual values of recharge. However, these time scales can be too coarse for the systems to which they are being applied and can lead to over- or underexploitation of groundwater. A numerical model of the Querença-Silves aquifer in Portugal is used to develop hypothetical scenarios in which abstraction rates for public water supply are adapted at various time-scales and are defined based on a percentage of the recharge which occurred during the previous period. The purpose is to understand the effects and feasibility of varying the temporal scale at which groundwater abstraction is modified in order to maximise sustainable yield and minimise freshwater losses. Results show that, for the Querença-Silves aquifer, reducing the time scale for which sustainable yield is defined allows for an increase in withdrawal volumes whilst maintaining the sustainability of the system. In fact, not reducing the temporal scale leads to an irretrievable loss of freshwater during recharge periods. Furthermore, predicted seasonal changes in rainfall for Portugal will make taking the temporal scale of the system into account more important, as the concentration of recharge into a shorter period will lead to faster depletion.