Contributions of increased rainfall and rising air temperature on hydrothermal dynamics in the permafrost of the Qinghai-Xizang Plateau

Driven by human activities and global climate change, the Qinghai-Xizang Plateau is experiencing a warming and humidifying trend. It significantly impacts the thermal-moisture dynamics in the active layer of the permafrost, which in turn affects the ecological environment of cold regions and the stability of cold region engineering. While the effect of air temperature on permafrost thaw has been well quantified, the processes and mechanisms underlying the thermal-moisture response of the permafrost to the combined effects of increased rainfall and rising air temperature remain contentious and poorly understood. A coupled model was applied to quantify the impacts of increased rainfall, rising air temperature, and their combined effects on the thermal-moisture dynamics in the active layer, considering the sensible heat of rainwater in the surface energy balance and water balance processes. The results indicate that the combined effects of warming and humidifying resulted in significant increases in surface net radiation and latent heat of evaporation, a more significant decrease in surface sensible heat, and a smaller impact of rainfall sensible heat, leading to an increase in surface soil heat flux. The combined effects of warming and humidifying also cause a significant increase in the temperature gradient-driven liquid water flux. The increase in the temperature gradient-driven liquid water flux is larger than that caused by warming alone, but smaller than that caused by humidification alone. Warming and humidification result in a smaller increase in soil moisture content during the warm season compared to that caused by rainfall increases alone. The thermal conductivity heat flux in the active layer increases significantly during the cold season but less than the effect of warming alone. The convective heat flux of liquid water increases noticeably during the warm season, but less than the effect of rainfall increases alone. Increased rainfall significantly cools the soil during the warm season, while both warming and humidification lead to more pronounced warming effects on the soil during the cold season than during the warm season. An increase in the average annual temperature by 1.0℃ leads to a downward shift of the permafrost table by 10 cm, while an increase in rainfall by 100 mm causes an upward shift of the permafrost table by 8 cm. The combined effects of warming and humidification results in a downward shift of the permafrost table by 6 cm. Under climate warming and humidification, the cooling effect of increased rainfall on permafrost is relatively small, with the warming effect of increased temperature remaining the dominant factor.