Critical role of groundwater discharge in sustaining streamflow in a glaciated alpine watershed, northeastern Tibetan Plateau

Abstract

As the hydrologic buffering capacity of glaciers diminishes on climate warming, groundwater stored in the glaciated alpine watersheds becomes an important source of streamflow, quantifying the groundwater contribution to this streamflow is significant for better predictions of the impact of rapidly disappearing glaciers on regional water resources. However, the role of groundwater in sustaining streams remains unclear. Here, we selected the upper Shule River Basin (USRB) on the northeast Tibetan Plateau (NETP) as a case to address this knowledge gap through a comprehensive study of geochemistry and stable isotopes data, the application of an end member mixing model and the baseflow hydrograph separation program (HYSEP). Our results indicate that even though the potential sources of streamflow exhibited distinct monthly differences during December 2012-December 2013, the groundwater was the dominant contributor to streamflow generation in the USRB. The groundwater contributed 45% to 100% of the monthly mean streamflow, and the annual mean value of 70%. By contrast, the glacier-snow meltwater and precipitation only contributed 12% and 18% of the annual discharge. The volume contributed by groundwater was calculated as 9.93 × 10⁸ m³, approximately six times higher than the input of the glacier-snow meltwater (i.e., 1.63 × 10⁸ m³). From 1954 to 2018, the volume of the groundwater discharge to the streamflow in the USRB continuously increased from 4.83 × 10⁸ m³/a (65% of total streamflow) in the 1950s to 10.34 × 10⁸ m³/a (71% of total streamflow) in the 2010s, an increase of 2.14 times. The retreating glacier, as well as increasing precipitation and temperature, were determined to be the main reasons for the increase in groundwater discharge to the streamflow. Our findings suggest that groundwater contribution is more important than was previously thought in the glaciated alpine watersheds on the NETP.

Keywords: Alpine watershed; Groundwater; Hydrological processes; Stable isotopes; Streamflow; Tibetan Plateau.