Variability of soil freeze depth in association with climate change from 1901 to 2016 in the upper Brahmaputra River Basin, Tibetan Plateau

Soil freeze depth (SFD) is necessary to evaluate the variations in water resource balances, surface energy exchanges, and biogeochemical cycles in the frozen ground regions, while the variability of regional SFD under climate changes remains unknown, mainly due to the insufficiency of effective observations. Significant changes of frozen ground related to water resources and the eco-environment have occurred in the Upper Brahmaputra River Basin (UBRB) (also known as the Yarlung Zangbo River) on the southern Tibetan Plateau, which is a key area where SFD monitoring is scarce. In this study, we adopted the Stefan equation to simulate the regional maximum SFD over the UBRB from 1901 to 2016 with monthly near-surface air temperatures from the University of East Anglia Climatic Research Unit (UEA-CRU) and the University of Delaware (UDEL). Observations from 11 national meteorological stations over the UBRB and 6 nearby were used to calibrate relevant parameters and evaluate the simulations. Studies showed that maximum SFD is positively correlated with air freezing index (AFI), ground surface freezing index (GFI), and snow depth (SD) and is negatively correlated with air thawing index (ATI), ground surface thawing index (GTI), mean annual air temperature (MAAT), and mean annual ground surface temperature (MAGST). Approximately 97.94% and 96.21% of the UBRB were covered with the frozen ground as simulated with the UEA-CRU and the UDEL, respectively. The maximum SFD calculated with the UEA-CRU and the UDEL decreased respectively by about 0.31 m and 0.15 m over the past 116 years. Simulations with both datasets demonstrated that no significant changes of SFD occurred in the past four 30-year periods (i.e., 1901–1930, 1931–1960, 1961–1990, and 1991–2016), while the maximum SFD calculated with the UEA-CRU was smaller than that with the UDEL. All of these features demonstrated a warming trend of frozen ground from 1901 to 2016 in the UBRB.