祁连山区黑河上游高山多年冻土区活动层季节冻融过程及其影响因素

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

祁连山区黑河上游多年冻土和活动层土壤水热过程基础观测数据薄弱,制约了黑河流域多年冻土区不同景观下垫面的水文功能研究以及多年冻土和活动层变化的水文效应研究.根据2013~2014年在黑河上游高山多年冻土区东支峨博岭北坡和西支冲积平原的活动层土壤温度、含水量观测资料,系统分析了活动层季节冻融过程、土壤水热动态及其影响因素.结果表明,高山多年冻土区气候条件以及地形地貌、植被、岩性和含水量等局地因素明显影响活动层季节冻融过程及其土壤水热动态,此外还受到积雪、水体和冬季逆温等因素的影响.峨博岭北坡地表温度年较差、活动层底板温度和年平均地温均明显低于黑河西支冲积平原同等海拔处.峨博岭北坡融化始日早、达到最大融化深度的日期晚、融化过程历时长、融化速率小;冻结（自地表向下）始日晚、冻结过程历时长、冻结速率小,完全冻结阶段历时长;在融化上升阶段和冻结下降阶段,活动层含水量变化速率明显较大.本文结果可为黑河流域多年冻土区不同景观下垫面的水文功能研究以及多年冻土和活动层变化的水文效应的辨识、模拟和预测提供基础资料和验证.

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.