Spatial Distribution Characteristics and Influencing Factors of Permafrost Active Layer Thickness in Tuotuo River Area, Qinghai–Tibet Plateau

The distribution and variation of active layer thickness (ALT) are crucial indicators for assessing the stability and environmental conditions of permafrost regions, which significantly impact regional hydrology, ecology, climate change, engineering construction, and disaster risk assessment. Based on the measured ALT data and Stefan equation, this study investigated the spatial distribution characteristics of ALT in the Tuotuo River region and explored the factors influencing its variability. The results showed that the ALT in the Tuotuo River area ranged from 0.15 to 5.18 m, with an average value of 2.65 m. The spatial distribution showed that the ALT was thinner in the southern region, which exhibited strong spatial heterogeneity, while the northeastern region generally had thicker ALT. Additionally, mountain areas tended to have thinner ALT, whereas plains showed thicker ALT. There was a good linear correlation between the simulated and measured ALT values, and the R2 was up to 80%. The ALT in the Tuotuo River area was mainly controlled by air temperature and surface water thermal conditions. Among all factors, soil water content was identified as the key determinant. Topographic factors influenced ALT distribution and variation mainly through their impact on soil water content.