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.

Surface energy budget and soil hydrothermal regime are crucial for understanding the interactions between the atmosphere and land surface. However, large uncertainties in current land surface process models exist, espe-cially for the permafrost regions in the Qinghai-Tibet Plateau. In this study, observed soil temperature, moisture, and surface energy fluxes at four sites in permafrost regions are chosen to evaluate the performance of CLM5.0. Furthermore, the soil property data, different thermal roughness length schemes, and dry surface layer (DSL) scheme are investigated. The results show that the soil property data is important for CLM5.0. The default scheme in CLM5.0 yields large errors for surface energy fluxes. The combination of the thermal roughness length and DSL scheme significantly improved the simulation of surface energy fluxes, especially for latent heat flux. The optimization of DSL scheme significantly improved soil temperature simulation and decreased the RMSE from 1.95 degrees C, 2.07 degrees C, 2.02 degrees C, and 2.95 degrees C to 1.34 degrees C, 1.35 degrees C, 1.35 degrees C and 2.29 degrees C in TGL site, respectively. The combination of the thermal roughness length and DSL scheme performed the best in shallow soil moisture, decreasing the RMSE from 0.136 m3 m- 3 to 0.049 m3 m- 3 in the XDT site but slightly enhancing the errors in middle soil. The interactions between surface energy and soil hydrothermal regime also discussed. However, the thermal roughness length and the DSL schemes are highly dependent on the condition of the underlying surface. Different schemes should be selected for different regions.

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.

Ecosystem responses to current global climate change can be predicted through experimental climate simulations. One such simulation method is the open-top chamber (OTC). The effects of OTCs on environmental factors are potentially complex, and recognizing the numerous interactions among these factors is crucial for the proper use of chambers. We studied the effects of OTCs on microclimatic factors including ambient temperature, relative humidity, soil temperature, and soil moisture. Plant abundance responses were also assessed. Our study involved the construction of 20 OTCs (1 m in diameter and 0.75 m in height; made of clear acrylic plastic) and 20 control plots on substrates with and without Sphagnum moss, at post-fire and logging sites of the transitional mixedwood-boreal forest in the southern part of James Bay region, Quebec. Experimental trials were also conducted to test the effects of OTCs on snowmelt in the Montreal region. Our results suggest that OTC treatment is most evident in terms of increased daytime maximum temperatures (2A degrees C to 3A degrees C), and cooler (up to similar to 2.4A degrees C), drier (up to 10% volumetric moisture content) soils. Advanced thawing of the insulating snow cover and exposure of soil in the OTCs to low spring temperatures appeared to prolong soil freeze and result in cooler soils. Earlier snowmelt probably also led to earlier onset and overall increased evaporation of meltwater in the OTCs, leading to drier soils. Plant abundance responses to OTC treatment differed depending on plant species. Overall, open-top chambers provide an effective and simple method of climate change simulation, but it is highly advisable that the complex interactive effects, both desired and undesired, are well understood and appreciated before using OTCs for experimental climate simulation.