From recharge deep uncertainty to desalination lock-in: propagation of groundwater recharge uncertainty into water-resources management and adaptation pathways in Macaronesian oceanic islands

Oceanic islands often rely on groundwater for up to 95% of freshwater supply, yet groundwater recharge remains one of the most uncertain components of island water budgets due to steep climatic gradients, volcanic aquifer heterogeneity, sparse monitoring and strong interannual variability. This uncertainty is frequently underrepresented in planning and can propagate into groundwater allocation rules and infrastructure decisions, potentially fostering maladaptive trajectories.

We analyse how deep uncertainty in recharge estimation propagates through the groundwater-management chain, from recharge assessment and water-balance calculations to abstraction limits, coastal salinization risk and long-term infrastructure planning. We quantify an “uncertainty cascade” using multi-island examples from the Canary Islands (Macaronesia). In El Hierro, recharge estimates reported in the literature span ~30–114 mm yr⁻¹ (CV ≈ 44%), while recent physically-based modelling constrained by evapotranspiration yields ~58 mm yr⁻¹ (≈15.6 hm³ yr⁻¹), implying a ~50% downward revision compared to commonly adopted values. For Gran Canaria, estimated renewable groundwater resources range from ~140 hm³ yr⁻¹ (1970s) to ~80 hm³ yr⁻¹ in current plans, while our assessment suggests substantially lower values (~40–63 hm³ yr⁻¹ depending on assumptions). In La Palma, our calibrated estimate indicates ~50.8 hm³ yr⁻¹ (2000–2020), yet desalination planning is already being advanced for agricultural supply reliability in highly stressed areas.

Within the GENESIS project framework, we further evaluate climate change as an additional pressure capable of inducing a dramatic reduction in recharge across island aquifers, aggravating overexploitation, accelerating seawater intrusion thresholds and reinforcing desalination dependence (lock-in). We discuss adaptation pathways aligned with the European Climate Adaptation Strategy, with a focus on ultra-peripheral (outermost) regions, where water systems are among Europe’s most climate-sensitive and likely to experience early impacts. We highlight how Nature-based Solutions (NbS) and reclaimed water management can reduce demand on groundwater, increase system robustness, and protect critical island water infrastructures. Ultimately, uncertainty-aware governance is essential for equitable adaptation and to ensure that no communities are left behind.