Since the 20th century, due to global warming, permafrost areas have undergone significant changes. The degradation of permafrost has complicated water cycle processes. Taking the upper Yellow River basin (UYRB) as a demonstration, this study discusses the long-term (1960-2019) changes in frozen ground and their hydrological effects with a cryosphere-hydrology model, in particular a permafrost version of the water and energy budget-based distributed hydrological model. The permafrost at the UYRB, with thickening active layer and lengthening thawing duration, has degraded by 10.8%. The seasonally frozen ground has a more pronounced intra-annual regulation that replenishes surface runoff in the warm season, while the degradation of permafrost leads to a runoff increase. The occurrence of extreme events at the UYRB has gradually decreased with the degradation of frozen ground, but spring droughts and autumn floods become more serious. The results may help better understand the hydrological impacts of permafrost degradation in the Tibetan Plateau.

Frozen soil undergoing freeze-thaw cycles has effects on local hydrology, ecosystems, and engineering infrastructure by global warming. It is important to clarify the hydrological processes of frozen soil, especially permafrost. In this study, the performance of a distributed cryosphere-hydrology model (WEB-DHM, Water and Energy Budget-based Distributed Hydrological Model) was significantly improved by the addition of enthalpy-based permafrost physics. First, we formulated the water phase change in the unconfined aquifer and its exchanges of water and heat with the upper soil layers, with enthalpy adopted as a prognostic variable instead of soil temperature in the energy balance equation to avoid instability when calculating water phase changes. Second, more reasonable initial conditions for the bottom soil layer (overlying the unconfined aquifer) were considered. The improved model (hereinafter WEB-DHM-pf) was carefully evaluated at three sites with seasonally frozen ground and one permafrost site over the Qinghai-Tibetan Plateau (the Third Pole), to demonstrate the capability of predicting the internal processes of frozen soil at the point scale, particularly the zero-curtain phenomenon in permafrost. Four different experiments were conducted to assess the impacts of augmentation of single model improvement on simulating soil water/ice and temperature dynamics in frozen soil. Finally, the WEB-DHM-pf was demonstrated to be capable of accurately reproducing the zero curtain, detecting long-term changes in frozen soil at the point scale, and discriminating basin-wide permafrost from seasonally frozen ground in a basin at the headwaters of the Yellow River.