Impacts of elevational variability of climate and frozen ground on streamflow in a glacierized catchment in Tibetan Plateau

In cold and high-elevation mountainous catchments, climate and landscape vary with elevation, which leads to elevational variability in runoff. The short-term variation and long-term change of climate would temporally and permanently alter the conditions of frozen ground and runoff characteristics at different elevation zones. In this study, a conceptually hydrological model is developed to investigate the responses of soil freeze-thaw and runoff processes to climate change from 1979 to 2013 in a glacierized catchment in the Tibetan Plateau (TP). Results show that our model can accurately reproduce the observed daily streamflow. In addition to rainfall (63.8%), meltwater from glacier (22.2%) and snowpack (14.0%) are also key contributors to streamflow, especially at high elevations. As temperature declines with rising of elevation, the elevation-runoff relationship depicts a convex formation with a runoff peak at the elevation of -5800 m. Below 5800 m, surface flow increases towards high elevations accompanied by the increase of glacier coverage, while groundwater flow reduces because the enlarged frozen ground areas inhibit the percolation of the infiltrated water. Above 5800 m, the runoff declines sharply as the ground changes to a perennially freezing condition. The long-term climate warming during 1979 - 2013 significantly increases annual runoff with a rate of 12.2 mm/10a. The increment in streamflow is primarily attributed to an increase in surface flow in the summer season when glacier meltwater increases at high elevations. Whereas the permafrost degradation enhances infiltrated water percolation and hence, groundwater flow in the low elevations and the low flow periods. Although climate warming benefits the local water resources availability during the historical periods, streamflow could be substantially decreased if the glacier vanished, which threatens the sustainability of the "water tower" over TP.