Stable Water Isotopes Improve Calibration and Flow Path Identification in Integrated Hydrological Models

Abstract Hydrological models often exhibit equifinality, where different parameter sets produce similar hydrometric outputs but alter internal catchment processes and partitioning. Incorporating new information amongst calibration targets helps narrow the range of viable models. While stable water isotopes have gained traction in rainfall‐runoff model calibration, their application in integrated surface‐subsurface hydrological models (ISSHM) remains limited. We test how combining hydrometric (streamflow Q, groundwater levels GWL) and isotopic (stream δ 18 O) targets affects parameter sensitivity, guides calibration, and informs flow path inference. Using the ISSHM HydroGeoSphere model for the sub‐arctic Pallas catchment, we applied a workflow combining global sensitivity analysis, uncertainty and automated parameter estimation with PEST++ across multiple target combinations. Sensitivity analysis showed δ 18 O alone produced patterns similar to those from combined targets (Q + GWL + δ 18 O), suggesting isotopes can independently constrain key system parameters. All calibration setups performed comparably for hydrometric outputs but diverged in internal storages and partitioning. Combining isotopic and hydrometric targets led to greater recharge via rapid deep percolation and yielded the narrowest parameter uncertainty. δ 18 O‐only calibration yielded the widest uncertainty bounds yet reproduced similar core internal dynamics, underscoring its standalone diagnostic power. Hydrometric‐only calibration inferred minimal recharge, increased soil storage, and produced ‐potentially misleading‐ intermediate uncertainty. These findings demonstrate that δ 18 O reveals internal catchment dynamics undetected by hydrometric data alone in GW‐SW modeling. Although stable water isotope information (in the form of δ 18 O) cannot fully resolve equifinality, its integration with hydrometric targets improves parameter identifiability and model realism. We therefore advocate wider adoption of δ 18 O in ISSHM calibration.