ABSTRACT The Mediterranean region is highly vulnerable to climate change (CC), with expected decreases in precipitation and rising temperatures leading to increased evapotranspiration and reduced water availability. The Tagus River Basin (TRB), shared by Spain and Portugal, is the Iberian Peninsula's third largest and most populated basin. Its upper section (UTRB) is especially critical, as it hosts 7.5 million people and supplies the largest inter‐basin water transfer in Spain. CC scenarios considered for the TRB management plan were assessed using a conceptual model—not able to reproduce its complex hydrological behaviour—for all Spain, jeopardising the reliability of the estimated impacts and requiring advanced methodologies to further evaluate future CC scenarios and to provide robust information for water resource management. In this study, we used a detailed and reliable SWAT+ model of the UTRB—calibrated considering the simulation of hydrological processes—together with up‐to‐date downscaled and bias‐corrected projections, to simulate and analyse four CC scenarios using 11 global climate models. Scenarios were evaluated at different spatiotemporal scales and considering their impact on numerous hydrological processes. Our results indicate that uncertainty related to climate scenarios and models is noticeably larger for the end of the 21st century, as the magnitude of the predicted impacts, which include a large reduction of water availability for all processes, especially under the most pessimistic scenarios. We found greater reductions in the UTRB than those identified in the Spanish administration's national‐scale study, which suggests that climate‐change impacts may be underestimated and that further study of CC effects on Spanish water resources is needed. The comprehensive evaluation of CC scenarios, and especially the use of a detailed hydrological model, provides a reliable estimation of the magnitude and uncertainty of CC impacts in this relevant basin, while also offering key methodological guidance for replicating this robust assessment in other basins.
Sánchez‐Gómez et al. (Sun,) studied this question.