Assessing Water-Security Threats in Iberia: Basin-Scale Projections of Drought Duration, Dry-Spell Frequency and Extreme-Rainfall Contribution

Hydroclimatic extremes in the Iberian Peninsula (IP) are projected to intensify under 21st-century climate change, with important consequences for water availability, agriculture, and infrastructure. This study quantifies spatial changes in key precipitation-based indicators, Consecutive Dry Days (CDD), frequency of extended dry spells (CDDn; spells >5 days), and the extreme-event contribution to total rainfall (R95pTOT), using ensemble means from nine CMIP6 global models downscaled with CHELSA v2.1 to ~1 km resolution. Historical conditions (1991–2010) and three future time slices (2041–2060, 2061–2080, 2081–2100) were evaluated for SSP1-2.6, SSP3-7.0 and SSP5-8.5. Analyses were performed at 30″ spatial resolution and aggregated by major Iberian basins to inform basin-scale governance and adaptation planning.

Baseline climatology exhibits a marked NW–SE gradient, with humid Atlantic basins (e.g., Galicia, western Pyrenees) receiving 1,600–1,900 mm yr⁻¹, while southeastern interiors often fall below 300–400 mm yr⁻¹. Historical R95pTOT is highest (50–60 %) in Atlantic-influenced catchments, indicating a large share of total rainfall concentrated in very heavy events, and lowest (10–20 %) in Mediterranean southern basins dominated by convective storms. CDD and CDDn likewise show strong spatial contrasts: Atlantic margins present short CDD (<30 days yr⁻¹) and relatively few long dry spells, whereas Guadalquivir, Guadiana and Segura basins already experience mean CDDs reaching 80 days yr⁻¹.

Projected changes reveal a coherent drying across most of Iberia and an intensification of rainfall extremes. Under high emissions (SSP5-8.5) by 2081–2100, ensemble mean annual precipitation declines of 25–40 % are simulated across key southern and interior basins (Guadalquivir, Guadiana, Segura), with CDD extending beyond 90 days yr⁻¹ and CDDn roughly doubling relative to the historical baseline, substantially elevating agricultural drought risk and reducing reservoir recharge and baseflow. Concurrently, R95pTOT increases by up to ~45 % in Atlantic-facing and mountainous catchments, indicating a shift toward more episodic rainfall where a greater proportion of annual totals is delivered by extreme events. The resulting hydroclimatic regime is therefore characterised by longer dry seasons punctuated by more intense, concentrated precipitation, escalating both drought persistence and flash-flood hazard.

These combined signals underscore the urgency of integrated basin-scale adaptation: enhancing water-use efficiency, optimising reservoir operations, expanding managed aquifer recharge, and implementing nature-based solutions to bolster soil moisture and reduce runoff. Embedding high-resolution, multi-indicator projections into transboundary governance and operational planning will be essential to improve resilience across the heterogeneous landscapes of the Iberian Peninsula.

Keywords: Iberian Peninsula; climate change; CDD; R95pTOT; CMIP6 ensemble; water security; adaptation.

Acknowledgements: This work is supported by National Funds by FCT – Portuguese Foundation for Science and Technology, under the projects UID/04033/2025: Centre for the Research and Technology of Agro-Environmental and Biological Sciences (https://doi.org/10.54499/UID/04033/2025) and LA/P/0126/2020 (https://doi.org/10.54499/LA/P/0126/2020).