From Watershed Science to Earth System Science: paradigm shift and disciplinary innovation in global governance

Watersheds are natural integrators of water, materials, energy, and information flows, and they have long served as the operational units for hydrology and environmental management. Yet under the accelerating pressures of the Anthropocene—climate non-stationarity, land-use intensification, engineered regulation, and socio-economic coupling—the reductionist tradition of decomposing a basin into isolated sub-systems has reached its limits. Persistent bottlenecks include scale mismatch between local processes and global drivers, fragmentation across spheres (water–air–rock–biosphere–human systems), and weak translation of heterogeneous data and model outputs into policy. This review therefore traces the evolution from classical Watershed Science (WS) to Watershed System Science (WSS) and further to Watershed Earth System Science (WESS), arguing for a paradigm that unifies mechanism discovery, prediction, and governance within a single evidence chain.We define WSS as a system-oriented turn that elevates multi-sphere coupling (“water–rock–air–biosphere–intelligence”), emergent behavior (nonlinearity, thresholds, lags), and human–nature coevolution at the basin scale. WSS reframes the basin as a complex adaptive system and shifts inquiry from single-process explanations to mechanism-rich representations that can diagnose how flow paths, residence time, connectivity, biogeochemistry, and human decision-making co-produce macroscopic patterns. WESS extends this logic into the Earth-system domain: under planetary boundary constraints, it seeks two-way coupling and consistent upscaling/downscaling between watershed, regional, and global representations, so that local disturbances (e.g., land conversion, reservoir operation) and global responses (e.g., climate feedbacks, carbon–water budgets) are expressed within a unified modeling and assessment architecture.Methodologically, the paper synthesizes an end-to-end workflow—Observation → Data Assimilation → Coupled Modeling → Scenario Evaluation → Governance Translation—that closes the loop between evidence and decision. Observation integrates air–space–ground platforms (satellites, UAVs, in-situ sensor webs) with tracers, isotopes, and eDNA to reveal water–biogeochemical coupling at high resolution. Ensemble Kalman/variational schemes then assimilate multi-source data to jointly constrain state variables (soil moisture, LAI, nutrient concentrations) and sensitive parameters (conductivity, reaction rate constants). On this foundation, modular coupling links basin hydrology and river-network water quality with land/vegetation and carbon modules in Earth-system models, while maintaining parsimony through standardized process libraries and interfaces. Scenario analysis leverages ensemble experiments to detect thresholds and early-warning signals under extremes and management options; outputs are mapped to decision-ready metrics—ecological flow, greenhouse-gas fluxes, nutrient loads, and resilience.The review highlights how technological advances (e.g., eDNA, digital twin basins) and global integration (embedding watershed datasets into Earth-system models) are pushing the field from explaining the past to designing futures, including proactive flood/drought risk governance, cross-border water cooperation, and SDG-aligned multi-objective trade-offs. A concise typology of model families (land/vegetation, basin hydrology and water quality, soil/crop GHG, riverine transport, and global couplers) is provided with notes on scale fitness and limitations, underscoring hydrology’s role as the coupling “spine” because flow paths, residence time, and connectivity regulate redox environments and reaction opportunities that set biogeochemical rates.Finally, we outline forward paths: (i) deepen cross-scale coupling and parameter transfer with consistent boundary and state spaces; (ii) prioritize key interfaces (riparian zones, wetlands, reservoirs, estuaries) where water–biogeochemistry–human interactions concentrate; (iii) institutionalize open benchmark datasets and joint assimilation for reproducibility; and (iv) co-design governance-facing indicators and platforms that translate science into implementable policy (from ecological flows to carbon–nutrient budgets and resilience diagnostics). Together, WSS and WESS reposition watersheds as “nervous nodes” of the Earth system and provide verifiable, operational decision support to meet water security, climate adaptation, and ecosystem restoration goals under planetary constraints.