Deciphering Intra‐Annual Isotope Fluctuations of River Water in an Arid‐Alpine Watershed: Source Discrimination and Evaporation Quantification

ABSTRACT As an indispensable resource underpinning the sustainable development of Golmud City and its surrounding regions, the Golmud River water plays a pivotal role in maintaining regional ecology, facilitating economic development and ensuring social welfare. To provide scientific guidance for local water resources management, the intra‐annual variations of the Golmud River water isotopes were investigated during 2019 to identify recharge sources and quantify evaporation impacts. Temporally, river water exhibited higher δ 2 H and δ 18 O values in July compared with other months. Spatially, δ 2 H and δ 18 O values of river water were the lowest in Xiaonanchuan (δ 2 H: −71.59‰ to −56.62‰, δ 18 O: −11.84‰ to −9.46‰), the highest in Yeniugou (δ 2 H: −66.2‰ to −46.79‰, δ 18 O: −10.98‰ to −6.52‰) and were between them below Nachitai. River water isotopes plotted below the local meteoric water line, and lc‐excess decreased from upper to lower reaches, indicating intensified evaporation along the flow path. Isotope‐altitude analysis showed the mean recharge elevation of river water exceeded 4700 m. MixSIAR modelling demonstrated that glacial meltwater (contribution: 29%–57.5%) and groundwater (contribution: 28.6%–46.8%) constituted primary recharge sources to river water for all the months, with enhanced precipitation contribution (24.2%) in July. Evaporation assessment revealed that the watershed‐scale evaporation loss averaged approximately 9%, remaining negligible in Xiaonanchuan while attenuating along the water flow in Yeniugou, with the highest value reaching 22% near Kunlun Lake and 25.8% in the downstream (east branch). The evaporation loss stability of the middle reach suggested the possibility of glacial meltwater recharge to river water through faults. The combined use of the Rayleigh fractionation model and lc‐excess was highlighted as a potential approach for river evaporation assessment. The findings advance the understanding of water cycling in arid‐alpine watersheds and provide critical insights for optimising regional water allocation.