Exploring future vulnerabilities of subalpine Italian regulated lakes under different climate scenarios: bottom‐up vs top-down and CMIP5 vs CMIP6

Study region: Lake Como watershed, between Northern Italy and Switzerland (Europe). Study focus: We analyse the potential vulnerabilities of existing water management practice in sub-alpine regulated lakes. We comparatively explore a wide range of inflow scenarios using a bottom-up approach, relying on synthetic alteration of observed climate drivers, and a top-down procedure, working with both CMIP5 and CMIP6 projections. We then run an ANOVA to backtrack components of variability in the projected lake inflows to key uncertainty sources. New hydrological insights for the region: Results show a general worsening of lake management performance under the top-down projections with respect to the equivalent bottom-up scenarios, with the CMIP6 ensemble yielding worst conditions than CMIP5. Moreover, the top-down, physically-based scenarios highlight non-linear patterns of climatic/hydrological variability that challenge historical management, while bottom-up projections overlook these trends. The results of the ANOVA demonstrate that at mid-century the choice of the general circulation model is the dominant factor influencing the variability, while climate patterns become more relevant toward the end of the century. Overall, our study provides numerical evidence of the ongoing and projected impact of climate change on sub-alpine regions and outlines how different inflow scenarios generation approaches result in different inflow variability and associated risk assessment.