Uncovering the mechanisms of seasonal river–groundwater circulation using isotopes and water chemistry in the middle reaches of the Yarlungzangbo River, Tibet

Abstract Alpine catchments are severely impacted by climate change and have been extensively studied from glaciological and hydrological perspectives. However, groundwater in such catchments is rarely mentioned, and the role of the groundwater system in sustaining streamflow in alpine catchments remains unknown. This study used an integrated framework with multi-tracer data (major ions and isotopes (2H, 3H, 18O, and Sr)) to identify seasonal water sources and to uncover the recharge–discharge mechanism of the river–groundwater system in the middle reaches of the Yarlungzangbo River (YR). The results show that surface waters collected in the dry season major ions exhibit higher solute concentrations and stable isotopes (H and O) are more enriched in river water than in the wet season. In addition, river δ18O values exhibit a distinct spatial variation that decreases and then increases along the flow direction in the dry season, but this is greatly attenuated during the wet season. Estimation results show that groundwater maintains the winter–spring baseflow (contributing 45.1%), including the discharge of deep circulating groundwater through macro structures, such as south–north oriented active tensile faults (contributing 19.6 ± 9.9%). During the wet season, precipitation (contributing 31.0 ± 3.6%) and tributaries (contributing 30.0 ± 8.2%) are the principal recharge end members of the river water. The rapid recharge of phreatic groundwater occurs in alpine regions through faults and fissures. Like the YR basin, the phreatic groundwater system is primarily maintained by modern meltwater and precipitation originating from oceanic moisture. The groundwater is younger than 20 years and recharges at a seasonal scale. Precipitation is likely the principal mechanism governing annual river flow and water level variations. Therefore, groundwater storage and total runoff in the YR Basin will increase greatly with climate warming in Tibet. Simultaneously, hydrological accidents are also likely to occur, requiring advanced monitoring and prevention measures.