The primary objective of this study was to quantify the contribution of glacial melt to total runoff in the Gaerqu River catchment, which is located in the source region of the Yangtze River, China. The isotope hydrograph separation method was used to separate glacier melt runoff from total runoff in the catchment. The degree-day method was used to investigate temporal variations in glacial melt runoff. The results showed that the contribution of glacial melt runoff to total runoff was 15.0%. The uncertainty of the separation was ± 3.7% at the confidence level of 95%. Glacial melt runoff was mainly generated in June, July, and August. The runoff coefficient was 0.23 for the catchment. Precipitation-induced runoff constituted 19.9% of the total precipitation, meaning that precipitation loss was >80% across the study period (a hydrological year). The Local Meteoric Water Line (LMWL) of the catchment was fitted as δ2H = 7.75 δ18O + 5.93. This line has a smaller slope and intercept than the Global Meteoric Water Line. The regression-lines for the δ18O and δ2H values of stream water indicated that evaporation was greater over the entire catchment than it was for the upstream region alone.

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