Evolution of runoff components and groundwater discharge under rapid climate warming: Lhasa river basin, Tibetan Plateau

The Tibetan Plateau (TP) is known as the world's third pole and an Asian water tower. Accelerated climate warming on the TP results in glacier retreat and runoff variations. Previous studies on the cryosphere of the TP mainly focused on the impact of climate warming on surface water, and studies on groundwater changes in response to climate warming are limited. In this study, we combine climate change, glacier, snow, surface water, and groundwater into a fully integrated surface-subsurface model based on HydroGeoSphere (HGS) for the Lhasa River basin (LRB), a catchment on the TP undergoing rapid glacier retreat. The model is used to predict river discharge and groundwater evolutions in the LRB from 2021 to 2100. Glacier melt runoff will reach its peak water by the 2060 s under the SSP2-4.5 scenario. Under the SSP5-8.5 scenario, temperature is predicted to rise sharply, leading to almost complete loss of the glaciers by 2100. The annual total runoff will increase consistently as rainfall runoff and groundwater discharge increase from 2020 to 2100 under the SSP2-4.5 and SSP5-8.5 scenarios. Consistent groundwater discharge can compensate the loss of glacier melt and snowmelt runoff to some extent. Groundwater discharge of the LRB will change substantially in response to climate warming in the future, especially in the Yangbajing sub-basin. Average hydraulic heads will increase by 0.130 m and 0.129 m per decade under the SSP2-4.5 and SSP5-8.5 scenarios, respectively. The results of this study will improve our understanding of the important role of groundwater in hydrological processes of alpine watersheds in response to climate change.