Tracer–aided method diagnoses hydrological model structural deficiencies and improves hydrological simulations in frozen-soil–affected catchments

Accurate hydrological simulation and credible climate-change projections in cold mountainous basins are hindered by complex phase transitions and strong cryospheric controls that exacerbate model equifinality and uncertainty in runoff component partitioning. This study advances a tracer-aided hydrological modeling framework by using stream-water stable isotopes to diagnose model structural deficiencies associated with frozen soils and to quantify how such deficiencies propagate into projections of hydrological sensitivity to climate change. We implement the tracer-aided Tsinghua Representative Elementary Watershed model (THREW-T) in a Tibetan Plateau cold catchment and compare two configurations: a baseline model lacking explicit frozen-soil processes (THREW-NoFS) and an enhanced version incorporating a simplified catchment-scale frozen-soil module with dynamically varying soil hydraulic properties (THREW-FS). While both configurations reproduce observed streamflow, isotope constraints expose a key limitation of THREW-NoFS: it cannot simultaneously capture baseflow dynamics and stream-water isotopic signatures, indicating missing freeze–thaw controls that effectively induce unrepresented seasonal variability in soil hydraulic behavior. Incorporating the frozen-soil module substantially improves the joint simulation of streamflow and isotopes and yields a more physically consistent runoff partitioning, characterized by reduced baseflow during dry seasons and increased subsurface runoff contributions during wet seasons. Frozen soils exert limited influence on annual discharge totals but markedly reshape runoff seasonality through altered surface–subsurface connectivity. Importantly, the isotope-informed structural correction changes projected climate sensitivity: both models suggest runoff decreases with warming and increases with precipitation intensification, yet THREW-NoFS produces systematically stronger sensitivities and tends to overestimate runoff responses because it provides more available water for evaporation and misrepresents surface–subsurface partitioning. These results demonstrate that tracer-aided hydrological models, when constrained by stable isotopes, offer a powerful pathway to diagnose frozen-soil impacts on model structure, reduce uncertainty in runoff component contributions, and generate more reliable projections of hydrological sensitivity to climate change in cryospheric basins.