Using stable hydrogen and oxygen isotopes to distinguish the sources of river runoff in the Naqu River Basin, Tibetan Plateau

The Naqu River Basin (NRB) is a crucial water conservation and ecological barrier area in the Nujiang River. Climate change has significantly altered its hydrological cycle, affecting water source composition. To investigate recharge dynamics in alpine regions, precipitation, surface water, and groundwater samples were collected from the NRB in Tibet, China, and analyzed for hydrogen and oxygen isotopes. Precipitation isotopes showed strong seasonal fluctuations, with the lowest values in winter and the highest in summer. The local meteoric water line (LMWL) had a lower slope and intercept than the global counterpart, indicating notable sub-cloud evaporation. Along the Naqu River, isotopic values varied spatially, becoming more enriched downstream. Lacustrine water had a higher isotopic composition than river water, reflecting intense evaporation in Cuona Lake. The river's main recharge sources include precipitation, glacial meltwater, groundwater, and lake water, with variations between freeze-thaw periods and along different sections. Meltwater and groundwater dominated recharge in upstream tributaries, particularly during the freeze ignition period, while lake water became a key source downstream.