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.

Identifying river water sources using end‐member mixing analysis in a subtropical monsoon basin China

“…For example, the average precipitation contribution and the monthly precipitation amount were 30% and 31.8 mm during the period from November 2020 to January 2021 (Figure 6), with the average monthly precipitation amount in June and July 2021 being 75.0 mm, and with the monthly values of streamflow discharge being uniform and low during these periods (Figure 6b). The similarity of the streamflow discharge in the winter and summer months, and the higher precipitation contribution and lower precipitation amount in the winter months compared to the values in summer months, observed in this study is consistent with the results of previous studies of the age and seasonal origin of river water (Jasechko et al, 2016; Xiao, Zhang, Wu, et al, 2022). Therefore, the consistency between model results based on isotopic and hydrochemical tracers supports the credibility of the hydrograph separations based on EMMA.…”

“…Studies based on stable isotopic techniques showed that winter precipitation may make a relatively large contribution to the river water, while the fraction of summer precipitation that becomes runoff is low compared to the fraction of winter or total precipitation that becomes runoff (Allen et al, 2019; Xiao, Zhang, Wu, et al, 2022). In this study, the EMMA results based on hydrochemical tracers showed that the summer precipitation seldom contributed to river water in June and July, especially in 2021, being only 1% in these 2 months.…”

“…In this study, the EMMA results based on hydrochemical tracers showed that the summer precipitation seldom contributed to river water in June and July, especially in 2021, being only 1% in these 2 months. The relatively high fraction of the creek water contribution in June and July (Figure 6c) may indicate the substantial inter‐seasonal carryover of precipitation that contributed to the river water (Allen et al, 2019; Xiao, Zhang, Wu, et al, 2022). The previous study of the Xiangjiang River basin showed that creek water had a relatively long renewal cycle, with over 80% of the creek water being more than several months old, and it is sourced from precipitation that fell several months previously (Xiao, Zhang, Wu, et al, 2022).…”

“…The relatively high fraction of the creek water contribution in June and July (Figure 6c) may indicate the substantial inter‐seasonal carryover of precipitation that contributed to the river water (Allen et al, 2019; Xiao, Zhang, Wu, et al, 2022). The previous study of the Xiangjiang River basin showed that creek water had a relatively long renewal cycle, with over 80% of the creek water being more than several months old, and it is sourced from precipitation that fell several months previously (Xiao, Zhang, Wu, et al, 2022). Thus, the precipitation in the spring rainy months may raise the soil moisture level or be stored in a shallow fractured rock aquifer, with the creek water being recharged after precipitation inputs and then contributing river water during the subsequent dry or rainless months (Figure 6c).…”

“…As a result of neotectonic influences, there are numerous fault structures (such as the Aiwanting fault and Erliban fault) in the Yuelu Mountains; the fault zones have abundant fractured rock aquifers and groundwater springs. Mountain creek water is exposed at fracture sites and is mainly recharged by soil water and fractured rock aquifer (Xiao, Zhang, Wu, et al, 2022).…”