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Since water resources of the Heihe River Basin are primarily in the form of surface runoff in the Qilian Mountains, identifying its sources and components is essential for researchers to understand water cycling and transformation in the basin. It will help to properly exploit water resources, and contribute to ecological environment construction. The paper uses the isotope data of hydrogen and oxygen in water and hydrochemistry data collected at a high altitude to trace the sources of surface runoff in Heihe River in rainy season and uses the three-component mixing model to estimate the contribution of each component to runoff. Results indicate that surface water consists of precipitation, melt water and groundwater, with precipitation being the primary component and contributing to 59%–64% of runoff. Melt water and groundwater account for 15%–25% and 12%–22%, respectively. Precipitation accounts for 60%, groundwater for 22% and glacial melt water for 18% of the outflow in the main stream of the Heihe River. The composition is of great significance for water cycling and conversion research as well as water resource evaluation and management.

Quantifying the inflow and outflow of groundwater is essential to understand the interaction between surface water and groundwater. It is difficult to determine these elements in relation to groundwater recharge and discharge to the river, because they cannot be directly measured through site specific study. The methods of isotope mass balance combining with water budget were used to quantify the groundwater recharge from and discharge to the Heihe River, northwest China. The mean isotope ratios of monthly monitoring data for one hydrological year were selected to be the isotope rations of end members in isotope mass balance. The results from the isotope mass balance analysis, incorporating with the 35-year hydrological data, suggest that about 0.464×109 m3/a of runoff flowing out Qilian Mountains is contributed to groundwater recharge (about 28% inflow of the Heihe River), while about 1.163×109 m3/a of runoff is discharged from groundwater in the middle reach of the river, which accounts for about 46% of river runoff in the basin. The analysis offers a unique, broad scale studies and provides valuable insight into surface water-groundwater interaction in arid area.