Active layer seasonal freeze-thaw processes and influencing factors in the alpine permafrost regions in the upper reaches of the Heihe River in Qilian Mountains

Abstract

Observational data on permafrost and active layer soil hydrothermal processes are extremely limited in the upper reaches of the Heihe River (URHHR) in Qilian Mountains. This lack acts as a bottleneck, restricting the research on the hydrological functions of different landscapes and the hydrological effects of the changes in the permafrost and active layer in the alpine permafrost regions of the Heihe River Basin. The active layer seasonal freeze-thaw processes and the soil hydrothermal dynamics and influencing factors were analyzed using soil temperature and water content observation data of the active layer established on the north slope of Ebo Mountains (NSEBM) in the east branch and in the alluvial plain in the west branch in alpine permafrost regions in the URHHR from 2013 to 2014. The results showed that climatic conditions in the alpine permafrost regions and local factors, such as topography, geomorphology, vegetation, lithology, and soil water content, evidently affected the active layer seasonal freeze-thaw processes and soil hydrothermal dynamics, and the main influence factors also contained snow, water, and winter temperature inversion. The annual surface temperature range (ASTR), mean annual ground temperatures (MAGTs) of the active layer, ground temperature at the bottom of the active layer (TTOP), and MAGT at the depth of zero annual amplitude were lower by 8.8, 0.6 to 1.7 (at depths of 5-77 cm), 0.7, and 0.7℃, respectively, on the NSEBM than at the same elevation in the west branch. Compared with the active layer at the same elevation in the west branch, the active layer onset date of soil thaw on the NSEBM occurred earlier. Moreover, the date reaching the maximum thaw depth occurred later, the duration of the seasonal thaw process was significantly longer, and the rate of the seasonal thaw process was lower. With the later active layer onset date of soil freeze from the ground surface downward, the duration and the rate of the seasonal freeze process on the NSEBM was longer and lower, respectively. Furthermore, the duration of the completely frozen stage on the NSEBM was longer. The active layer onset date of the thaw-rising stage, the relative completely thaw stage, the freeze-fall stage, and the completely frozen stage on the NSEBM lagged significantly from top to bottom, whereas that of the last three stages were evident at the same elevation in the west branch. The rate of change in the active layer soil water content at depths of 20-60 cm at the thaw-rising stage and the freeze-fall stage was both significantly higher on the NSEBM. In addition, the active layer soil water content at the depths of 20-60 cm from the mid- to late completely frozen stage in the beginning of the year to the early thaw-rising stage was lower on the NSEBM. These results were mainly due to the effects of local factors, such as the soil particle composition, dry density, ice content, and organic matter content on the active layer soil water dynamics. This study provides basic data that identify, simulate, and predict the hydrological functions of different landscapes and the hydrological effects of the changes in the permafrost and active layer of the Heihe River Basin. Our study can also provide a reference for the study of seasonal freeze-thaw processes and influencing factors in permafrost regions with different climate types, such as other alpine areas in Western China, the north-eastern Qinghai-Tibet Plateau, Northeast China, and even in the Arctic or sub-Arctic.