ABSTRACT Peninsular Spain presents a remarkable climatic and topographic diversity creating major challenges for sustainable management of water resources. This complexity is further compounded by intense anthropogenic pressures on river systems and increasing climatic uncertainty, raising critical questions about long‐term water availability. Our study investigates whether Spanish watersheds are gaining or losing water by quantifying discrepancies between topographic and effective watershed areas using the Effective Catchment Index (ECI). We analysed 381 catchments from the BULL database over three decades (1990–2020), comparing results derived from three climate datasets: AEMET (observations), ERA5‐Land and EMO‐1 reanalysis. Consistent results between AEMET and ERA5‐Land indicate that 73% of basins are losing water (with effective areas less than half their topographic extent), and only 2% are gaining (with effective areas more than double their topographic size). EMO‐1 produced markedly different outcomes (52% losing, 14% gaining), revealing substantial uncertainties inherent to certain reanalysis products. Spatial correlation and bivariate clustering analyses identified catchment size, topographic area, and the proportion of lakes and reservoirs as the main factors explaining ECI variability, accounting for over 60% of the observed patterns. Overall, our findings demonstrate that water losses in most Spanish catchments are predominantly controlled by structural and anthropogenic factors rather than climatic conditions alone. The study underscores the importance of considering groundwater–surface water interactions and effective catchment area in large‐scale hydrological assessments, providing a new framework for improving water resource evaluations under changing climatic and human pressures.
Segura‐Méndez et al. (2026) studied this question.