<p>The confluence of escalating anthropogenic demands and climate change-driven hydrological alterations has accelerated the Mediterranean region’s vulnerability to water scarcity. This study applied the Soil Water Assessment Tool (SWAT+) model to simulate hydrological behavior and assess impacts of climate change in the El Torcón and Picadas reservoir catchments, located within the Tagus River basin in central Spain. Advanced SWAT + capabilities, incorporating reservoir operations and water allocation modules, were utilized to simulate regulated (non-natural) flow transfers from the catchments. Model calibration and validation employed an innovative multi-criteria framework that combined hard and soft calibration techniques by integrating statistical and process-based metrics to ensure not only statistically robust models but also realistic representation of hydrological processes. A multi-variable calibration strategy was adopted using observed reservoir inflow, storage, and streamflow data to enhance the models’ overall performance and reliability. Climate impacts were analyzed for the medium-term (2041–2070) and long-term (2071–2100) projections under the Shared Socioeconomic Pathways (SSP2–4.5 and SSP5–8.5) scenarios using 11 downscaled General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Modeling in Phase 6 (CMIP6). The main findings of the study were: (1) Projections indicate declining precipitation (up to − 23%) and increasing potential evapotranspiration (up to + 23%) in both catchments, thereby intensifying hydrological deficits, particularly under SSP5–8.5. (2) Long-term projections under SSP5–8.5 scenario indicate substantial decline in water availability across both catchments, with reservoir inflows declining by − 53% in El Torcón and − 41% in Picadas, implying an accelerating transition towards an arid hydrological regime. Based on these findings, urgent implementation of adaptive and climate-resilient water governance strategies might be necessary to mitigate the effects of these climate change-driven hydrological disruptions. This study demonstrates the robustness of SWAT + as an effective tool for assessing future water resources availability, providing evidence-based decision-making, and informing long-term water resource planning amidst climate uncertainty, particularly in vulnerable, water-stressed regions. </p> Graphical Abstract <p></p> <p>This graphical abstract presents a concise visual summary of the research by offering an overview of the study area, methodologies, and core findings, thus serving as a pivotal entry point for readers. The visualization starts with the two study areas, followed by the model configuration, then calibration and validation, then presents climate change simulation, and at the bottom is a visual summary that illustrates the future climate change impacts on key hydrological variables for both catchments. The research focused on two Mediterranean reservoir catchments (El Torcon and Picadas) located in Tagus basin in central Spain. The SWAT+ model was employed by integrating its advanced capabilities such as reservoir operations and water allocation modules to simulate the catchments natural and regulated hydrological behavior and climate projections, ensuring a robust assessment of future water security. Model calibration and validation were conducted using multi-criteria (hard and soft), multi-variable approach implemented through the R package SWATRunR, yielding statistically satisfactory performance while also realistically representing the catchment processes well. Climate change was simulated using 11 General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Modeling in Phase 6 (CMIP6) under moderate (SSP2-4.5) and high (SSP5-8.5) emission scenarios, downscaled for the study areas. Projections for the medium-term (2041–2070) and long-term (2071–2100) were compared against the historical baseline (1981–2010) using an ensemble of the GCMs. Future projections revealed declining trends in total reservoir inflow in both catchments driven by declining precipitation and increasing potential evapotranspiration due to increased warming. Similar declining trends were observed in direct and groundwater flows, signaling a transition toward a more arid hydrological regime, further exacerbating the existing water stress in the region. The findings highlight the growing vulnerability of Mediterranean reservoir catchments to climate-driven aridification, with significant implications for future water resource planning.</p>