Classifying vulnerability to sea-level rise of coastal shallow groundwater at the national scale for New Zealand

• NZ coastal aquifers classified by water table dynamics: head- or flux-controlled. • Head-controlled systems are frequent in low-elevation coastal zones, susceptible to saltwater intrusion. • Flux-controlled systems are dominant, susceptible primarily to groundwater rise. • National study (60% flux-controlled) refines global estimate of 75% head-controlled. A national-scale assessment provides a comprehensive classification of near-surface unconfined coastal groundwater systems in New Zealand and predicts their vulnerability to sea-level rise. Using high-resolution national datasets, the research applies the water table ratio framework to classify aquifers into head-controlled (topography-limited) or flux-controlled (recharge-limited) systems based on their water table dynamics. Head-controlled aquifers are identified as more vulnerable to saltwater intrusion and the exposure of shallow groundwater, leading to subsurface and infrastructure risks. In contrast, flux-controlled systems are primarily susceptible to water table rise and subsequent groundwater flooding. The findings indicated that while flux-controlled systems are more prevalent by catchment count along the New Zealand coastline (64% of catchments), head-controlled systems dominate in terms of land area (56% of coastal catchment area) and are characteristic of most low-elevation coastal zones (71%). In terms of length of coastline, the majority was flux-controlled (60% of coastline length). This national-scale assessment significantly refines prior coarser global predictions, which estimated that 75% of the NZ coastline was head-controlled. Location-dependent insights are essential for informing effective national water management and enhancing resilience against climate change impacts on New Zealand’s coastline.