Application of Stable Isotope in Hydrologic Analysis of a Rainfall-Runoff Event in the Xin’an Jiang River Basin

Abstract:

Stable isotopes abundance of hydrogen (δ²H) and oxygen (δ¹⁸O) are very sensitive to environmental changes and can help understand the complex recharges between surface water and groundwater. However, details of variability in the different components of water supply to watersheds are generally missing. We present here evaluation of isotopic changes in small watershed (0.19 km²) in the upstream of Xin’an River, Anhui province, China based on daily sampling of rainfall events and runoff (river and soil water). The δ²H and δ¹⁸O of different water sources in the watershed were aimed in order explore interactions between different parts of the hydrological processes. The collected water samples were analyzed for δ²H and δ¹⁸O using Picarro L-2120i analyzer at a precision of 1.0 and 0.1 ‰, respectively. The results indicated higher abundance of ²H and ¹⁸O in precipitation than those in river. Additionally, the content of the heavy isotopes also decreases in soil water with depth, while surface soil water was enriched because of evaporation. The response of the watershed to the changes in δ²H and δ¹⁸O varied, where water in upstream gained more heavy isotopes rapidly, while in the downstream part the enrichment happened after a few days. This feature suggests a different but still strong hydraulic connection between surface water and groundwater in the small watershed tested here. Accordingly, variability of δ²H and δ¹⁸O should be carefully evaluated on a local scale before application in transport system of large rivers and exchange with groundwater.