Paradigm shift in water-energy nexus: Cognitive reconstruction and methodological innovation from resource dependency to system coupling

Energy and water are tightly coupled resources whose interactions shape economic development, ecological security, and climate resilience. Yet, existing studies about water-energy nexus remain fragmented, often focusing on isolated mechanisms, single scales, or sector-specific interventions, limiting their capacity to inform integrated governance. This review synthesizes the concept evolution, methodological progress, and application pathways of water-energy nexus research, and critically identifies four progressively interconnected research domains: resource dependency accounting, interaction network characterization, dynamic evolution assessment, and coordinated optimization and decision-making. To facilitate the methodological integration, we propose the novel Evaluation-Optimization Pathway for Water-Energy Systems (EOP-WES), a modular, extensible, and interpretable analytical framework that unifies dispersed modeling practices into a coherent pathway. EOP-WES provides clear, stepwise guidance on how to move from nexus dependency diagnosis to causal interaction mapping, scenario-based risk assessment, and ultimately cross-sector optimization and actionable strategies. To illustrate its applicability, we present a basin-scale conceptual example for the Yellow River Basin, demonstrating how EOP-WES can be instantiated in complex, multi-resource nexus system. Upon the EOP-WES framework, we outline key research challenges and future directions, emphasizing (i) water-energy co-management under decarbonization policies, (ii) vulnerability identification and adaptive scheduling under climate extremes, and (iii) cross-sectoral, cross-scale pathways for knowledge translation and policy integration. Collectively, these contributions provide a unified paradigm for advancing theoretical innovation, methodological integration, and policy relevance in sustaining water and energy security under deep uncertainty and transformative change.