Stable isotopes of surface water and groundwater in a typical subtropical basin in south‐central China: Insights into the young water fraction and its seasonal origin

Abstract The young water fraction (Fyw) can be reliably determined in heterogeneous river basins, and the seasonal origin index (SOI) can be used to determine the relative contributions of winter versus summer precipitation to runoff. Fywand SOI are also important descriptors of how a basin controls runoff generation and the transport of contaminants and nutrients. In this study, high‐frequency sampling of precipitation, river water, creek water, and shallow groundwater was conducted in the Xiangjiang River basin, in south‐central China, from January 2010 to December 2012. The samples were subjected to stable isotope measurements to determine the SOI and Fywof surface water and shallow groundwater. The principal findings are: (1) The δ2H variations of the river water, creek water, and shallow groundwater were driven by the input of precipitation but were attenuated during runoff confluence processes. Also, the low lc‐excess values of the river water indicated a strong evaporative effect. (2) 21.4% and 19.7% of the river water and shallow groundwater were younger than 1.5 and 1.7 months, respectively, indicating a quicker renewal cycle than for the creek water, which had a fraction of 8.0% younger than 2.6 months, which originated from the adjacent mountains. (3) Winter precipitation was more prevalent in the river water and creek water, indicating the substantial inter‐seasonal carryover of precipitation that supplied the runoff. The average SOI value of the shallow groundwater was near zero, indicating the mixing of the precipitation in different seasons that supplies the shallow groundwater. Our findings emphasize the importance of vegetation and soil conservation in maintaining the inter‐seasonal carryover of water storage held by soil, plants, and rock fractures, and the necessity to assess the risks of streamflow contamination because of the rapid renewal cycle of river water in this flat and intensively cultivated landscape.