Hydrothermal processes of near-surface warm permafrost in response to strong precipitation events in the Headwater Area of the Yellow River, Tibetan Plateau
Introduction
With an average elevation exceeding 4,000 m above sea level (a. s. l.), the Tibetan Plateau (TP) becomes the southernmost large expanse of permafrost away from the main Eurasian permafrost zone. Therefore, the permafrost on the TP is warm and thermally unstable, for half of the ground temperatures at the depth of zero annual amplitude (TZAA) (usually at 10–15 m) are within −1.0 and 0 °C (Wu et al., 2010). Based on the thermal stability indicated by TZAA, the permafrost on the TP is specifically classified into thermally stable (<−3.0 °C), metastable (−3.0 to −1.0 °C), unstable (−1.0 to −0.5 °C), and very unstable (>−0.5 °C) (Cheng, 1984, Jin et al., 2011). With respect to this, more than half permafrost on the TP is warm, thermally unstable, and sensitive to climate changes (Wu et al., 2010, Zhao et al., 2017, Xu et al., 2017). Warm permafrost on the TP degraded seriously over the past decade or two, which is evidenced in the aspects of rising in permafrost temperatures, increase in active layer thickness (ALT), and formation of taliks, and so forth (Cheng and Wu, 2007, Guo et al., 2018, Luo et al., 2018, Ran et al., 2018, Wu et al., 2015, Wu and Zhang, 2008, Zhou et al., 2016).
Permafrost is one of the major components of terrestrial freshwater on the Earth’s surface due to the storage of a large amount of ground-ice, which could be roughly estimated based on cryogenic strata from hundreds of boreholes over the TP (Li et al., 2008, Zhao et al., 2010). With the permafrost layer being a medium of limited permeability, active groundwater circulations in permafrost regions could be categorized into suprapermafrost, intrapermafrost, and subpermafrost waters, which occur respectively above, within, and beneath the permafrost layer (Woo, 2012). Suprapermafrost water mainly exists in the active layer and is greatly affected by seasonal freeze-thaw cycles, normally recharged by meteoric water with precipitation as the main water resources (Zhu et al., 2017). The variation of the active layer and the frequent and periodic hydrothermal processes are key to modulating the near-surface biogeochemical cycles and supporting the plant growth and microbial activities (e.g., Fisher et al., 2016, Guglielmin et al., 2008). Intrapermafrost water occurs in taliks within the permafrost, which is unfrozen even though the water can be below 0 °C if it is high in concentrations of dissolved solids (Woo 2012). On the eastern Tibetan Plateau with patch permafrost, rich liquid water was observed to present in frozen sands and gravels with the subsurface temperature between −0.5 and 0 °C (Wang, 1990, Cheng and Jin, 2013). Confined by impervious perennially frozen layer, subpermafrost water with a temperature above 0 °C is found below permafrost (Woo 2012).
Permafrost degradation likely alters the hydrological cycles and the stability of cold eco-environments (Li et al., 2018, Li et al., 2018, Zhang et al., 2017b, Zhang et al., 2018, Guglielmin et al., 2012, Guglielmin et al., 2014). Melting of ground-ice in association with permafrost degradation is considered as one of the principal contributors to the increased groundwater storage on the TP in the context of climate change (Zhang et al., 2017). This is especially serious for warm permafrost with TZAA > −1.0 °C. In warm and thin permafrost areas, liquid water converted from the melting of ground-ice likely participates in local subsurface hydrological cycles and increases groundwater storage. With the rising of soil temperatures above freezing point (assumed to be 0 °C or so), the permeability of near-surface warm permafrost weakens with the transformation of ground-ice into liquid water. Accordingly, meltwater or suprapermafrost water infiltrates through the perennially frozen layer, causing the transformation of aquitard (frozen) into the aquifer (thawed) in thermally unstable permafrost regions (e.g., Harris, 2001). As a result, the surface, supra-permafrost, intra-permafrost, and subpermafrost waters are conditionally interconnected through the pores, fissures, or dilation cracks, owing to the rupturing of frozen soils (Cheng and Jin, 2013). In turn, the waters percolating in fractures may accelerate the degradation of permafrost (Gruber and Haeberli, 2007). Permafrost degradation is estimated to persist in the future (Zhang and Wu, 2012, Guo et al., 2012), further contributing to changes in hydrological cycles, and even the performance and security of “Asian Water towers” (e.g., Cheng and Jin, 2013, Nair et al., 2017, Zhu et al., 2017, Bosson et al., 2012, Zhou et al., 2020).
Permafrost makes up 61.0% of the total area of the Three-River Headwater Area (the headwater areas of the Yangtze, the Yellow, and the Lancang-Mekong rivers) (TRHA), northeastern TP (Luo et al., 2016). The up-to-date investigations demonstrated that most permafrost TZAA in the TRHA were within −1.0 and 0 °C and experienced an average warming trend of 0.2 °C (10a)−1 in the 2010s (Luo et al., 2018). Due to the increase of soil permeability resulting from the degradation of permafrost, the recessions of groundwater and the lowering of regional water table remarkably prevail in the Headwater Area of the Yellow River (HAYR), a critical zone belonging to the TRHA (Cheng and Jin, 2013, Zhang et al., 2009, Jin et al., 2009). As a result, the eco-environment had seriously deteriorated, which is characterized by land desertification, exacerbation of alpine ecosystems, and even dry-up of main streams of the Yellow River (Liang et al., 2010, Zhang et al., 2003). In the context of climate shifting from warm-dry to warm-wet (Shi et al., 2006), the changes of precipitation were manifested by increases in the amount and unevenly daily and seasonal distribution (e.g., Cao and Pan, 2014). The increased heavy precipitation events now are the major contributor to the increase of annual rainfall-induced erosion in the headwater areas of the Yellow and Yangtze rivers (Wang et al., 2017), which is presumed to intensify in the coming decades. However, it remains unclear how the permafrost associated with seasonal hydrothermal processes react to heavy and super-heavy precipitation, even though a few investigations examined the impacts of soil permeability and the seasonal freeze-thaw processes in response to the precipitation events in warm permafrost (e.g., Wang, 1990, Cheng and Jin, 2013, Zhu et al., 2017).
To enrich the knowledge on the impacts of climate change on permafrost, related investigations of permafrost hydrology are urgently required concerning the impermeability of underlying perennially frozen soils. In this study, the HAYR dominated by warm permafrost was chosen to examine the hydrothermal responses of warm permafrost to heavy and super-heavy precipitation events. Our aims were to: (1) characterize the water-heat variations of seasonal freeze-thaw processes at two sites, (2) compare the permeability of seasonally and perennially frozen soils and, (3) examine the effects of precipitation events on the hydrothermal processes of active layer and near-surface permafrost in the HAYR.
Section snippets
Study sites
The HAYR is a representative warm permafrost region characterized by the mosaicked occurrence of continuous and discontinuous permafrost and seasonally frozen ground (Fig. 1) (Luo et al., 2020). Temperature measurements from drilled boreholes demonstrate that permafrost in the HAYR is warm (>−1 °C) and thin (<100 m), and degrades slowly for extremely warm ones (with TZAA between −0.5 and 0 °C) in the context of climate warming (Li et al., 2016, Luo et al., 2018). The landforms in the HAYR are
Water content on the ground surface
The ground surface (at the depth of 0.05 m) responds immediately to the heavy and super-heavy precipitation events, including the super-precipitation day on 2 July 2015 and the heavy precipitation week from 23 to 25 June 2014, which are evidenced by abrupt increases in soil water content. In 2014, after two heavy precipitation events on 24 (16.2 mm) and 25 June (20.4 mm), soil water content increased rapidly from 11.8% to 17.5% on 25 June and to 25.8% on 26 June at HRQ1, and from 29.2% to 34.8%
Hydrothermal processes of the active layer
We found that the −0.1 °C isotherm matched well with the relatively high soil water content (Fig. 6). The common definition of ALT with maximum seasonal penetration of the 0 °C isotherm (e. g., Fisher et al., 2016, Wu et al., 2015) may insufficiently capture the ground surface freezing and thawing regimes for sites like HRQ1 and HRQ2. The realistic freezing point of soil water is commonly depressed toa slightly subzero temperature under the influences of a variety of factors (Woo, 2012,
Conclusions
The influence of precipitation on the hydrothermal processes of the near-surface warm permafrost was investigated at two sites, HRQ1 and HRQ2, at a small basin in the Headwater Area of the Yellow River, Tibetan Plateau. Although the climates of HRQ1 and HRQ2 are similar, the variation amplitude of soil temperature of HRQ1 with saline soils is smaller than that of HRQ2 with stratified ground-ice.
During the thawing period, especially from late-May to October, a relatively high content of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is jointly supported by the National Key Research and Development Program of China (2017YFC0405701), the Strategic Priority Research Program of the Chinese Academy of Sciences, China (Grant No. XDA20100103), and National Natural Science Foundation (NSF) of China (Grant No. 41671060).
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2023, CatenaCitation Excerpt :The reconstructed fine-scale long-term continuous permafrost degradation processes filled the gap between discontinuous observation and the requirements for global warming impact research. The method used in this research overcame the previous limitations with respect to short time period in field observations (Farbrot et al., 2013), large-scale problem in empirical equations based on air and soil surface temperature (Luo et al., 2019; Riseborough et al., 2008; Yin et al., 2016), relative shallow soil simulating problem in the land surface model (Chen et al., 2018; Luo et al., 2013; Yu et al., 2014), and hydrologic processes in the pure permafrost model (Luo et al., 2020a; Goodrich, 1982; Oelke et al., 2003; Zhang et al., 2006a). Many indexes directly or indirectly described the features of permafrost and SFG, including MAAT, MAGT, TTOP (Wang et al., 2018; Zou et al., 2017), ALT (Zhang et al., 2020), PA, ground surface temperature (GST) (Hu et al., 2020), and lower altitude limit of permafrost (Cao et al., 2021; Ran et al., 2018).