Isotopic evidence for the moisture origin and composition of surface runoff in the headwaters of the Heihe River basin

We investigated the moisture origin and contribution of different water sources to surface runoff entering the headwaters of the Heihe River basin on the basis of NECP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) re-analysis data and variations in the stable hydrogen and oxygen isotope ratios (delta D and delta O-18) of precipitation, spring, river, and melt water. The similar seasonality in precipitation delta O-18 at different sites reveals the same moisture origin for water entering the headwaters of the Heihe River basin. The similarity in the seasonality of delta O-18 and d-excess for precipitation at Yeniugou and Urumchi, which showed more positive delta O-18 and lower d-excess values in summer and more negative delta O-18 and higher d-excess values in winter, indicates a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter. Higher d-excess values throughout the year for Yeniugou suggest that in arid inland areas of northwestern China, water is intensively recycled. Temporal changes in delta O-18, delta D, and d-excess reveal distinct contributions of different bodies of water to surface runoff. For example, there were similar trends for delta D, delta O-18, and d-excess of precipitation and river water from June to September, similar delta O-18 trends for river and spring water from December to February, and similar trends for precipitation and runoff volumes. However, there were significant differences in delta O-18 between melt water and river water in September. Our results show that the recharge of surface runoff by precipitation occurred mainly from June to mid-September, whereas the supply of surface runoff in winter was from base flow (as spring water), mostly with a lower runoff amount.