Permafrost is mostly warm and thermally unstable on the Tibetan Plateau (TP), particularly in some marginal areas, thereby being susceptible to degrade or even disappear under climate warming. The degradation of permafrost consequently leads to changes in hydrological cycles associated with seasonal freeze-thaw processes. In this study, we investigated seasonal hydrothermal processes of near-surface permafrost layers and their responses to rain events at two warm permafrost sites in the Headwater Area of the Yellow River, northeastern TP. Results demonstrated that water content in shallow active layers changed with infiltration of rainwater, whereas kept stable in the perennially frozen layer, which serves as an aquitard due to low hydraulic conductivity or even imperviousness. Accordingly, the supra-permafrost water acts as a seasonal aquifer in the thawing period and as a seasonal aquitard in the freezing period. Seasonal freeze-thaw processes in association with rain events correlate well with the recharge and discharge of the supra-permafrost water. Super-heavy precipitation (44 mm occurred on 2 July 2015) caused a sharp increase in soil water content and dramatic rises in soil temperatures by 0.3-0.5 degrees C at shallow depths and advancement thawing of the active layer by half a month. However, more summer precipitation amount tends to reduce the seasonal amplitude of soil temperatures, decrease mean annual soil temperatures and thawing indices and thin active layers. High salinity results in the long remaining of a large amount of unfrozen water around the bottom of the active layer. We conclude that extremely warm permafrost with T-ZAR (the temperature at the depth of zero annual amplitude) > 0.5 degrees C is likely percolated under heavy and super-heavy precipitation events, while hydrothermal processes around the permafrost table likely present three stages concerning TZAR of 0 degrees C.

Long-term thermal effects of air convection embankments (ACEs) over 550-km-long permafrost zones along the Qinghai-Tibet railway were analyzed on the basis of 14-year records (2002-2016) of ground temperature. The results showed that, after embankment construction, permafrost tables beneath the ACEs moved upward quickly in the first 3years and then remained stable over the next 10years. The magnitude of this upward movement showed a positive correlation with embankment thickness. Shallow permafrost temperature beneath the ACEs decreased over a 5-year period after embankment construction in cold permafrost zones, but increased sharply concurrent with permafrost table upward movement in warm permafrost zones. Deep permafrost beneath all the ACEs showed a slow warming trend due to climate warming. Overall, the thermal effects of ACEs significantly uplifted underlying permafrost tables after embankment construction and then maintained them well in a warming climate. The different thermal effects of ACEs in cold and warm permafrost zones related to the working principle of the ACEs and natural ground thermal regime in the two zones. (c) 2018 American Society of Civil Engineers.