Spatiotemporal variations in frozen ground and their impacts on hydrological components in the source region of the Yangtze River

The source region of the Yangtze River (SRYR), located on the eastern Tibetan Plateau, is an essential part of the Asian Water Tower and plays an important role in the downstream water resources. Significant changes in frozen ground caused by increases in air temperature have been widely reported in the past several decades, which has greatly affected regional runoff. This study evaluated the spatiotemporal variations in frozen ground and hydrological components by utilizing a geomorphology-based eco-hydrological model (GBEHM) and investigated the reasons for runoff changes based on the Budyko framework. The results showed that the area with an elevation range of 4700-4800 m located in the permafrost region was the main source area of runoff generation from 1981 to 2015. Compared with the permafrost region, the seasonally frozen ground (SFG) region had a larger ratio of annual evapotranspiration to annual precipitation, although the aridity indices in the two regions were very similar. From 1981 to 2015, the mean value of the maximum frozen depth of SFG (MFDSFG) decreased by 12.3 cm/10 a and the mean value of the active layer thickness (ALT) of permafrost increased by 4.2 cm/10 a. The annual runoff in the SFG region decreased, while that in the permafrost region increased. Runoff change was more sensitive to precipitation change in the higher altitude regions that were mainly covered by permafrost than in the lower altitude regions that were mainly covered by the SFG, while the evapotranspiration change in the transition zone was more sensitive to climate change. An abrupt change in the annual runoff time series was detected in 1989, 2004, and 2004 in the SFG region, the permafrost region and the entire SRYR, respectively, and the annual runoff change from period 1 (1981 to change point) to period 2 (change point + 1 to 2015) were - 25.7 mm, 33.8 mm and 25.8 mm respectively. Frozen ground degradation contributed changes of -15.0 mm, - 8.8 mm and -11.6 mm to the annual runoff in the SFG region, the permafrost region and the entire SRYR, respectively. This result implied that frozen ground degradation had a negative impact on regional runoff in the SRYR. These findings deepen our understanding of frozen ground and its hydrological changes and are helpful for water resource management in the SRYR.