Abstract 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 °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 TZAA (the temperature at the depth of zero annual amplitude) >−0.5 °C is likely percolated under heavy and super-heavy precipitation events, while hydrothermal processes around the permafrost table likely present three stages concerning TZAA of 0 °C.

中文翻译：

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青藏高原黄河源区强降水事件响应的近地表暖多年冻土热液过程

摘要 青藏高原（TP）特别是一些边缘地区的多年冻土大多是暖温不稳定的，在气候变暖下容易退化甚至消失。因此，永久冻土的退化导致与季节性冻融过程相关的水文循环发生变化。在这项研究中，我们调查了青藏高原东北部黄河源区两个温暖多年冻土区近地表多年冻土层的季节性热液过程及其对降雨事件的响应。结果表明，浅层活动层的含水量随着雨水的入渗而发生变化，而在常年冻结层中保持稳定，该层由于导水率低甚至不透水而起到隔水层的作用。因此，超永久冻土层水在解冻期作为季节性含水层，在冻结期作为季节性透水层。与降雨事件相关的季节性冻融过程与超永久冻土层水的补给和排放密切相关。超强降水（2015 年 7 月 2 日发生 44 毫米）导致土壤含水量急剧增加，浅层土壤温度急剧上升 0.3-0.5°C，活动层提前半个月解冻。然而，更多的夏季降水量往往会降低土壤温度的季节性幅度，降低土壤年平均温度和解冻指数，使活动层变薄。高盐度导致活性层底部周围长期残留大量未冻结的水。