[Spatial Variation and Environmental Significance of δ¹⁸O and δD Isotope Composition in Xijiang River]

The H and O isotope composition of the Xijiang water was investigated on 54 samples collected from the mainstreams and main tributaries in June 2014 and January 2015.It was found that in the Xijiang river, there was a remarkable linear relationship between δ¹⁸ O and δD. This relationship was more significant in the dry season. In both seasons, the ¹⁸ O and D values distributed along the meteoric water line, which indicated that precipitation was the source of the Xijiang River. Due to the direct water recharge through karst forms (i.e. sinkholes, vertical shaft and funnel) to the underground river, the d-excess value was higher in the rainy season. While during the dry season, water recharge came from the storage in fissures and pores. Affected by altitude and evaporation effect, along the mainstream Hongshui River-Xunjiang and tributary Youjiang-Yujiang reach, the δ¹⁸ O and δD values were gradually approaching positive value in general. However, along the short tributary Guijiang reach, the altitude difference was not dramatic, and the discharge was less, the same phenomenon was not observed. By establishing a linear regression model between δ¹⁸ O and altitude, it revealed that the change rate of Xijiang water with altitude was -0.44‰·(100 m)^-1 in the rainy season, and -0.45‰·(100 m)^-1 in the dry season. The result reflected the height effect of the precipitation, which has a considerable meaning for the study of hydrological cycle in a river basin.