Temporal and spatial variations of the active layer along the Qinghai-Tibet Highway in a permafrost region

Using monitored active layer thickness (ALT) and environmental variables of 10 observation fields along the Qinghai-Tibet Highway in permafrost region of the Qinghai-Tibetan Plateau (QTP), a model for ALT estimation was developed. The temporal and spatial characteristics of the ALT were also analyzed. The results showed that in the past 30 years ALT in the study region increased at a rate of 1.33 cm a−1. Temperatures at the upper limit of permafrost and at 50 cm depth, along with soil cumulative temperature at 5 cm depth also exhibited a rising trend. Soil heat flux increased at a rate of 0.1 W m−2 a−1. All the above changes demonstrated that the degradation of permafrost happened in the study region on the QTP. The initial thawing date of active layer was advanced, while the initial freezing date was delayed. The number of thawing days increased to a rate of 1.18 d a−1. The variations of active layer were closely related to the permafrost type, altitude, underlying surface type and soil composition. The variations were more evident in cold permafrost region than in warm permafrost region, in high-altitude region than in low-altitude region, in alpine meadow region than in alpine steppe region; and in fine-grained soil region than in coarse-grained soil region.