Computing a Groundwater Sustainability Index for coastal aquifers in Portugal and California to foster sustainable groundwater management

Sustainability Indices can be useful to quantify objective groundwater management strategy outcomes, particularly across regional scales and when local groundwater budget data is not readily available. Previous studies have used performance indicators to evaluate surface water systems, and their application to groundwater is expanding to address water availability concerns. Here, a groundwater sustainability index (GSI) is computed across coastal aquifer systems in Portugal and California using reliability (REL), resilience (RES), and vulnerability (VUL) performance indicators. Aquifers in these Mediterranean climate zones are susceptible to inter-annual and seasonal water storage fluctuations linked to climate forcings and drought. Piezometric levels in the selected aquifers in Portugal (Leirosa-Monte Real and Campina de Faro) and California (Napa and Santa Barbara), spanning a period from 1989 to 2019, are analyzed using a point-wise approach to provide an index-per-piezometer. The computation exposes that the resilience indicator is heavily influential in setting an aquifer system's overall sustainability classification. However, the most significant output from the GSI is a clear indication of how well (or poor) a specific aquifer can withstand drought conditions that occur in both California and Portugal throughout the 30-year span of this study. Lastly, comparing indices with different priorities (performance indicators), such as sustainability and exploitive use (including the Water Framework Directive’s River Basin Management Plan’s Water Exploitation Index (WEI+)) can help identify aquifer systems that may need an immediate policy, conservation, or mitigation interventions, and others that may be self-sustaining for a longer period of time. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.