Comparative Hydrological Modeling of Snow-Cover and Frozen Ground Impacts Under Topographically Complex Conditions

Abstract. In cold regions, snow and frozen ground significantly influence hydrological processes, but understanding these dynamics remains limited due to insufficient data. We aimed at advancing process understanding and model capabilities, departing from the existing Gridded Xinanjiang (GXAJ) model framework and developing i) the Gridded Xinanjiang-Snow cover model (GXAJ-S) considering snowmelt and ii) the Gridded Xinanjiang-Snow cover-Seasonally Frozen ground model (GXAJ-S-SF) taking into account both snowmelt and freeze-thaw cycles. The models were calibrated to daily runoff data (2000–2010; calibrating also the snowmelt module to snow depth data) to reproduce runoff (2011–2018) from the middle and upper reaches of the Yalong River located in the topographically complex and seasonally cold zone of the Qinghai-Tibet Plateau. The results showed the relevance of considering not only snowmelt impacts, but also frozen ground impacts, as reflected in a clearly better GXAJ-S-SF model performance compared to both other model variants. In particular, the GXAJ-S-SF model output demonstrated that the presence of seasonal frozen ground (SFG), considerably increased surface water runoff (by 39–77 % compared to the two models that neglected SFG) during the cold months, while reducing interflow and groundwater runoff. Additionally, the GXAJ-S-SF model results showed a significantly reduced soil evapotranspiration. These results emphasize multiple and considerable impacts of SFG on runoff generation in mountainous areas. This modular approach has great potential for integration into other hydrological models and application in cold mountainous regions, where accounting for climate-driven SFG changes could significantly enhance future hydro-climatic assessments and predictions, including downstream water resource impacts.