Thermal Characteristics and Recent Changes of Permafrost in the Upper Reaches of the Heihe River Basin, Western China

To investigate the thermal characteristics and dynamics of permafrost as well as seasonally frozen ground over the upper reaches of the Heihe River Basin (URHR), an observation network with 14 boreholes was established during 2011–2014. The in situ measurements indicated mean annual air temperature that ranged from −5.2 to −2.3°C at the monitored elevation range of ∼3,600–4,150 m, and mean annual ground surface temperature that ranged from −1.3 to 1.7°C during 2013–2017. The mean annual ground temperature at 16- to 18-m depth ranged from −1.71°C on the high (>4,000 m above sea level) north facing slope to about 0-C around areas near the lower limit of permafrost. Active layer thickness at the monitored sites varied significantly with the range of 0.77–4.90 m during 2011–2017, and maximum frozen depth in seasonally frozen ground was about 5 m. Permafrost thickness was between ∼136 m and less than 10 m. Both permafrost and seasonally frozen ground were found to be subject to serious warming during the measured period in the URHR. This study provides new quantitative insights for permafrost and seasonally frozen ground in the URHR.