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Seasonal Variation of δ~(18)O in River Water in the Upper Reaches of Heihe River Basin and Its Influence Factors

WANG Ning-lian,ZHANG Shi-biao,PU Jian-chen,HE Jian-qiao,JIANG Xi,WU Xiao-bo(State Key Laboratory of Cryospheric Sciences,Cold and Arid Regions Environment and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou Gansu 730000,China)  
During the period of May 2006 through May 2007,precipitation and river water samples were collected at Yingluoxia,Qilian and Zhamashi Hydrological Stations and other sites in the upper reaches of the Heihe River.By analysis of δ18O in these samples,it was found that the variation of δ18O in river water displayed seasonal characteristics with high values in summer and low values in winter.These characteristics were mostly related with the variation of δ18O in precipitation in the Qinlian Mountains.A high correlation exists between the values of monthly mean δ18O in river water multiplied by monthly mean discharge and that of monthly precipitation-weighted mean δ18O in precipitation multiplied by monthly precipitation amount at Qilian Hydrological Station.This indicates that precipitation is the basic supply source for the mountainous runoff.Moreover,the "altitude effect" in δ18O in precipitation was found in the middle of Qinlian Mountains,where the upper reaches of the Heihe River is situated.At the three hydrological stations,the mean δ18O in river water was lower than δ18O in precipitation over the study period.This implied that the mountainous runoff was formed in high elevations.Based on the mean δ18O in river water at Yingluoxia Hydrological Station,which is just located at the foot of the Qilian Mountains,and the "altitude effect" in δ18O in precipitation,it is estimated that the mountainous area ranging from 3 350 m a.s.l.to 4600 m a.s.l.,which corresponds to the sub-alpine shrub-grassland vegetation zone and cold-desert alpine meadow vegetation zone,is the major area for the formation of the mountainous runoff.
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