Uncovering Systemic Dynamics through an Integrated WEFE Nexus Index across 21st Century Futures

Achieving sustainability under accelerating climate and socioeconomic pressures requires moving beyond siloed sectoral management toward a system-thinking approach. The water-energy-food-ecosystem (WEFE) Nexus offers a holistic lens, yet most applications remain conceptual, short-term, or treat ecosystems as external constraints. This study operationalizes the WEFE Nexus by embedding ecosystems as a coequal, quantified pillar through a hydrologic-regime-based method, since streamflow is a master variable shaping riverine ecosystem health. Long-term foresight is incorporated via dynamically downscaled climate projections and Shared Socioeconomic Pathways within a coupled water and energy systems (WEAP-LEAP) model. Applied to the semiarid Sakarya Basin in Türkiye, the framework evaluates three future periods (2020-2030, 2055-2065, and 2090-2100) across seven subbasins. Results show systemic trade-offs: municipal water security remains high (>90%), but ecosystem integrity and renewable energy goals are consistently compromised. Overall, WEFE Nexus Index values (0.53-0.86) show significant spatial disparities, with arid upstream regions consistently underperforming. Strikingly, SSP2 (business-as-usual) and SSP5 (fossil-fueled growth) yield nearly identical outcomes, underscoring the systemic unsustainability of current trajectories. This framework advances nexus assessment from theory to practice by integrating reproducible metrics, scenario planning, and spatial modeling, creating a practical tool for developing adaptive and resilient sustainability strategies.