Isotopic characteristics of precipitation, groundwater, and stream water in an alpine region in southwest China

The hydrological cycle has been enhanced under the warmer climatic conditions on the southeastern edge of the Tibetan Plateau, China. To investigate the complex hydrological system in the study area, δ¹⁸O and δ²H were measured on waters in a mountainous region—Huangbengliu watershed. The stream water tends to be isotopically more depleted in ¹⁸O and ²H than precipitation and groundwater, but more enriched than meltwater. A clear seasonality with higher isotopic values of stream water in summer and lower values in winter is evident, coinciding with the timing of the peak discharge. The correlation between δ¹⁸O and δ²H in stream water shows that the isotopic signature of precipitation is well preserved in stream flow, and the evaporation during the stream flow is only minor. The isotopic composition of stream water shows a decreasing trend from upstream to downstream, which results from tributary water inputs with heavy-isotope-depleted water. According to an isotopic mass-balance model, the fraction of meltwater inputs over the total stream flow ranged from 25.5 to 61.8 %. The study demonstrates that the ice–snow melting and tributary inputs are the prevailing mechanisms regulating stream isotope hydrology in alpine regions on the edge of the Tibetan Plateau.