Model diagnostic analysis in a cold basin influenced by frozen soils with the aid of stable isotope

Understanding the hydrological processes on the Tibetan Plateau (TP) is an important scientific question under climate change. The frequent multiphase transfer exacerbates the complexity of hydrological processes on the TP, which brings equifinality problem to hydrological models and causes large uncertainties in quantifying the contributions of runoff components. Tracer-aided hydrological models are helpful for improving model performances and have been adopted in cryospheric regions, but the influence of frozen soil has yet to be considered. This study adopted the tracer-aided hydrological model THREW-T (Tsinghua Representative Elementary Watershed, Tracer-aided version) in a typical cold basin with widespread frozen soil on the TP. The model structure was diagnosed with isotope by identifying the influences of frozen soil. A simplified catchment-scale frozen soil module was incorporated into the model. Results showed that: (1) The THREW-T model cannot simultaneously simulate baseflow and stream water isotope well. The imbalance of simulations on two objectives could be attributed to the influence of frozen soil, resulting in seasonal variation of soil-related parameters, which was not considered in the model. (2) Incorporating the frozen soil module significantly improved the balance of baseflow and isotope simulation, simultaneously producing low baseflow and high contribution of subsurface runoff during wet seasons. (3) The frozen soil had little influence on the annual streamflow, but changed the runoff seasonality by reducing baseflow during dry seasons and increasing subsurface runoff during wet seasons. The frozen soil module was still simplified, and further work is needed to improve the physical representation of soil freeze-thaw process. This study highlights the value of tracer-aided hydrological modeling method on diagnosing model structure by identifying the influence of specific processes such as frozen soil.