Impacts of winter warming and permafrost degradation on water variability, upper Lhasa River, Tibet

Watersheds in frozen ground have been influenced by climate change for centuries, particularly in the last two decades when permafrost degradation has accelerated and intensified. This degradation speeds up water transfer, increases soil moisture, improves agricultural productivity, and affects the ecological environment. It is important to evaluate the effects of these impacts on hydrology to develop sustainable water resources management in cold regions. This study evaluated changes in both winter air temperature and streamflow regime for a selected drainage basin at low latitude but high altitude following large scale permafrost degradation in the Tibetan Plateau. The non-parametric Mann–Kendall test was used to identify trends in both winter air temperature and streamflow. The results showed significant upward trends in winter flow but not summer runoff. The trend tests indicated that change points of winter streamflow and temperature occurred in 1985 and 1983, respectively. Comparison of winter flow duration for two 16-year periods (1976–1991 and 1992–2008) showed a significantly upward trend in winter streamflow. Increases in most winter flows varied from 16% to 24%, while the increase in low flows was much greater. There were reliable positive correlations between the October and November temperatures and the following monthly discharges. Both the temperature rise and discharge increase at start of the frost period can greatly increase drainage of subsurface water in February, when the maximum variability occurred. The total increases in winter flow are increasing as a function of time, with significant changes occurring since the early 1980s. However, it is yet unclear if the watershed has seen the full effects of the permafrost degradation over the Tibetan Plateau.