Spatiotemporal characteristics of freezing and thawing of the active layer in the source areas of the Yellow River (SAYR)

Based on the analysis of data on temperatures and moisture of soils in the active layer at four different permafrost sites in the source areas of the Yellow River (SAYR) in 2010–2012, the freeze–thaw processes of soils in the active layer were compared and contrasted for understanding the spatiotemporal variations. At the four studied sites, the thickness and mean annual temperature of permafrost are different. The temperatures at the top of permafrost (TTOP), i.e., the maximum depth(s) of seasonal frost and/or thaw penetration, are −1.9 °C at the Chalaping site (CLP), −0.9 °C at the site on the southern bank of the Zhaling Lake (ZLH), −0.4 °C at the Maduo Town site (MDX), and 1.1 °C at the site on the northern bank of the Eling Lake (ELH). Differences in the mean annual ground temperature of permafrost and TTOPs may be responsible for the differentiations in the freeze–thaw processes of soils in the active layer. With rising TTOPs, the ground thawing started earlier: CLP in early June, ZLH in late May, MDX in early May, and ELH in mid-April, while the freezing began later: CLP in early October, ZLH in early to mid-October, MDX in mid-October, and ELH in the mid- to late October. With increasing TTOPs, the freeze-up periods for permafrost sites were shortened: 202 days at CLP, 130 days at ZLH, 100 days at MDX, and the period of complete thaw was 89 days at ELH. At the CLP and ZLH sites, the two-directional ground freezing (downwards from ground surfaces and upwards from the permafrost table) and thawing finished in the same year, but the ground freezing at the MDX continued to the end of the next January, with very slow freezing rates in the end. At the ELH site, ground freezing kept on until early May when thawing began on the surface, and upward and downward thawing became increasingly stable in late June to early July. At each site, with rising TTOPs, the downward freezing accelerated in comparison with the upward freezing, and with an increasing proportion of downward frozen depth, and with the larger ratios of freezing to thawing duration. In summary, the patterns of thawing and freezing processes in the active layer in the SAYR differ from those in other parts of the Qinghai–Tibet Plateau to a noticeable extent.