Coupled Snow and Frozen Ground Physics Improves Cold Region Hydrological Simulations: an Evaluation at the upper Yangtze River Basin (Tibetan Plateau)

Cryosphere plays an important role in cold-region (e.g., Tibetan Plateau) hydrological processes under climate change, and thus cryosphere physics should be carefully represented in the hydrological modeling. In this study, based on the Water and Energy Budget-based Distributed Hydrological Model (WEB-DHM), we have further improved the cold-region hydrological processes, by incorporating the enthalpy-based coupled snow and frozen ground physics (hereinafter WEB-DHM-SF), and comprehensively evaluated at the upper Yangtze River Basin under widespread snow cover and frozen ground. The model was calibrated and validated at the basin scale using observed discharge at Zhimenda station during 1981-2016 and simulated discharge with high accuracies. The model also successfully reproduced the basin-wide daytime and nighttime land surface temperature (LST) as well as snow depth in basin-averaged time series and spatial distributions. At the point scale, the model successfully reproduced soil moisture and temperature profiles at Tuotuohe based on available yearlong observations. Furthermore, experiments were designed to investigate model improvements and indicated that the incorporation of the three-layer snow scheme with correct consideration of incident solar radiation attenuation among snow layers resulted in lowered snow melting and reduced discharges. Additionally, the incorporation of frozen ground physics with a correct description of soil water phase changes rendered the modeled soil moistures and temperatures more accurate in the cold season. Through rigorous evaluations of model performance, the incorporation of coupled snow and frozen ground physics demonstrates significant promise for improving cryosphere hydrological simulations in river basins on the TP.