With the ongoing effects of climate warming and wetting, the impact of precipitation on freeze-thaw processes in frozen ground is gaining significance. Nevertheless, the influence of precipitation during different freeze-thaw stages on the thermal regime of shallow soil remains poorly understood on seasonally frozen ground (SFG).
Here, based on the data observed at the Madoi and Maqu sites on SFG from 2014 to 2018, we analysed the response of soil hydrothermal characteristics and freeze-thaw duration to the precipitation during different freeze-thaw stages. The relationship between soil hydrothermal characteristics and precipitation was weak on an annual scale.
Interannual differences in the start and end of the freezing stage (FS) are mainly caused by variations in precipitation, which directly affect the soil liquid water content at both sites. An additional 28 mm of precipitation during the same two-month period encompassing FS postponed the start of freezing by 27 days and shortened the duration of FS by 14 days at Madoi site.
Precipitation differences were minimal at Maqu site, the FS showed less variability. Furthermore, the contrasting responses between Madoi and Maqu can be attributed to differences in soil properties. The low soil ice content during the completely frozen stage (CFS) can be attributed to either low precipitation during the FS or high snowfall during CFS at Madoi site. Lower air and soil temperature after the beginning of soil freeze resulted in the higher soil ice content at Maqu site.
The prolonged snow covers delayed soil thawing at the two sites. Compared to the FS, the thawing stage (TS) was more susceptible to the influence of snow cover due to the melting and infiltration of snow. Different snow melting patterns can significantly affect the soil thawing process.
Introduction
The Tibetan Plateau (TP) holds the distinction of being the largest and most extensive plateau globally. The energy and water dynamics of TP exert far-reaching impacts on the formation and evolution of China’s and even Asian climate system. The source region of Yellow River (SRYR) is located in the northeast of TP, with elevations ranging from 2459 m to 6253 m SRYR lies in the transitional zone between seasonally frozen ground (SFG) and
permafrost. Frozen ground plays an important role in the cryosphere and serves as a critical forcing factor for land surface processes. Climate change is expected to alter the regime and extent of frozen ground. Research over the past few years has actively investigated changes in frozen ground. Air temperature is widely recognized as the primary driver of permafrost thaw and SFG dynamics. Studies in the SRYR reveal rising permafrost temperatures, thickening active layers, and a shift from permafrost to SFG in marginal zones.
Concurrently, SFG exhibits delayed freezing onset, earlier thawing, and reduced maximum frozen depth. However, emerging evidence suggested that precipitation, particularly under the “warming-wetting” climate trend, plays an increasingly significant role in modulating freeze-thaw processes. Precipitation alters the soil hydrothermal properties through changes in water content.
The ability of soil to conduct heat and store energy is highly dependent on soil moisture, which in turn influences the overall energy exchange with the ground and thereby affects soil freeze-thaw processes. The role of precipitation has been considered in studies of soil temperature and freeze-thaw process based on linear fitting and machine learning due to the correlation between precipitation and soil freeze-thaw characteristics.
The influence of precipitation on freeze-thaw processes in frozen ground varies across different regions. Several studies have specifically examined the impact of summer precipitation on the hydrothermal characteristics and freeze-thaw processes of the active layer in permafrost, yet findings across different regions have been inconsistent Study on permafrost at high latitudes has indicated that increased summer rainfall is associated with deeper thawing at various sites, with an average increase of 0.7 ± 0.1 cm of thaw per additional centimeter of rain.
Simulated studies suggest that a 100 mm increase in summer precipitation could lead to a decrease of 0.35 m in active layer thickness on TP. Continentality influences warming or cooling effects of heavy rainfall events on permafrost, as well as the responses observed between different observation depths within the soil profile. In permafrost region of the TP’s semi-arid high altitudes, heavy summer rainfall has been observed to exert a cooling effect on the active layer, while simultaneously warming effect on the underlying permafrost body.
SFG and permafrost exhibit distinct thermal responses to climate forcing. Unlike permafrost, which retains a permanently frozen layer year-round, SFG undergoes complete thawing during warm seasons. This absence of a permafrost layer makes SFG particularly sensitive to seasonal changes in precipitation. The study found that enhanced summer rainfall on TP leads to warming the permafrost but cooling SFG remarkably. Surface energy fluxes and hydrothermal characteristics are more responsive to increased precipitation than to decreased precipitation on SFG of TP.
The temporal distribution characteristics of annual precipitation also play a crucial role in its impact on frozen ground. An increase in year-round precipitation on TP results in a cooling effect on active layers during the frozen season, while exerting a warming effect during the thawed season. In the SRYR, increased summer precipitation has been found to cool permafrost, thereby reducing seasonal fluctuations in soil temperatures, decreasing mean annual soil temperatures, lowering thawing indices, and resulting in thinner active layers.
In cold regions, precipitation during spring, autumn, and winter often falls as snow. The influence of snow on seasonally frozen ground is complex and multifaceted. The insulating effect of snow cover is particularly embodied in the middle-frozen period. In years with less snow, prolonged daily freeze-thaw cycles have been observed in surface soil during spring on the SFG of SRYR.
The duration of snow cover plays a significant role in permafrost. Study suggested that long-lasting snow cover during early summer and early snow accumulations in fall reduce the active layer thickness. The result demonstrates that increased precipitation during cold and warm seasons exerted opposite effects on permafrost across the SRYR. In the vast majority of permafrost within SRYR, the cooling effect of warm season wetting outweighs the warming effects of cold season wetting.
However, in transitional zones with unstable permafrost, the warming effect of cold season wetting becomes relatively dominant, leading to accelerated permafrost degradation. This indicates that different types of frozen ground exhibit distinct responses to the warming and cooling effects induced by precipitation during cold and warm seasons.
Spatial analysis of precipitation variability reveals an increasing gradient from northeast to southwest in SRYR. Researches indicate that a significant rise in precipitation of the SRYR in recent years, characterized by pronounced decadal and spatial variations. Over the past 50 years, annual, spring, summer, and winter precipitation exhibited an increasing trend, while autumn precipitation has shown a decrease.
most precipitation occurs from May to September, although precipitation from October to April has increased rapidly. Additionally, extreme precipitation events have shown an increasing trend at high elevations and a decreasing trend at lower elevations in SRYR. The seasonal variability of precipitation across SRYR ultimately alters soil hydrothermal conditions, leading to modifications in soil freeze-thaw processes.
Most of the aforementioned studies on the mechanism of precipitation’s effects on frozen ground have primarily focused on permafrost, with limited mention given to SFG. Furthermore, to mitigate the influence of snow cover, previous studies have predominantly focused on summer precipitation, which has less impact on SFG devoid of a permafrost layer.
Precipitation impacts vary across different freeze-thaw stages, but current studies often aggregate annual or seasonal data, obscuring stage-dependent mechanisms. In this study, the data at two field sites with significant precipitation differences, located in the SRYR on the SFG in the past five years were analyzed. The aim is to quantify how precipitation during specific freeze-thaw stages alters shallow soil hydrothermal regimes on SFG, and assess the implications of climate wetting for SFG degradation and surface-atmosphere feedbacks.
Section snippets
Freeze-thaw stage of shallow soil
Four stages of soil freeze-thaw period were divided by the method of Section 2.2.4 in this study. Shallow soil at Madoi site undergoes freeze-thaw processes for approximately half of the year. From 2014 to 2018, the soil freeze-thaw period at the depth of 5 cm lasted the longest in 2015/16, totalling 218 days, while the shortest duration occurred in 2017/18, lasting 175 days. The earliest occurrence of soil freezing was on September 26, 2015. Notably, 2015/16 also recorded
Conclusion and discussion
This study aimed to investigate the effects of precipitation on the soil hydrothermal characteristics and freeze-thaw process during different freeze-thaw stages at Madoi and Maqu sites in SRYR on the SFG:
At Madoi site, the highest recorded precipitation from 2014 to 2018 occurred in 2018/19, totaling 481.5 mm, Madoi experienced its lowest precipitation in 2015/16 with 303.6 mm. The differences in precipitation between different years at Maqu site were smaller than Madoi site.
News Courtesy : Sciencedirect