The stable oxygen isotope composition of precipitation (δ18Op) in southern China is considered as a valuable proxy of climatic conditions. However, their interpretations have been controversial. In this study, based on the observed and simulated data (isoGSM2) on oxygen isotope composition of precipitation, the linkage between summer precipitation (P) and δ18Op in the Dongting Lake basin and their possible influencing factors were investigated. The results indicate that the interannual variation of summer δ18Op is consistent with that of annual δ18Op. They both show a significantly negative correlation with the summer P, suggesting that the stable isotope composition in precipitation may be considered as a proxy of summer precipitation in the Dongting Lake basin. Statistically, the amount effect and circulation effect are significant in the isotope composition of precipitation in the basin. Based on either the observed data in Changsha or the simulated data for the basin, the local amount effect appears more important than large‐scale circulation for δ18Op during extreme summers. These results can potentially improve the reconstruction of paleoclimate in the East Asian monsoon region. Further study is needed to determine the contribution of local and large‐scale factors to the oxygen isotope composition of precipitation and to quantify the integral rainout along the moisture transporting paths.

The study on precipitation isotope variation can potentially improve the understanding of weather processes, regional water cycle and paleoclimate reconstruction in the subtropical monsoon region. Based on the measured stable isotope composition in precipitation (δ18Op) and daily precipitation from January 2010 to December 2021 in Changsha of the subtropical monsoon region of eastern China, the δ18Op variations, amount effect and local meteoric water line (LMWL) were analyzed and compared on daily, monthly and annual time scales, as well as under different precipitation intensities. The results showed that, on the daily time scale, δ18Op was significantly and negatively correlated with precipitation in the study area. Influenced by subcloud evaporation, small precipitation events (≤5 mm/d) could change the rainout level of precipitation isotopes. There were significant differences in the slope and intercept of the LMWL on different time scales, in different seasons and under different precipitation intensities. On the daily and monthly time scales, the slope and intercept of the LMWL in the cold half of the year were significantly smaller and larger than those in the warm half of the year, respectively, and the slope and intercept of the LMWL increased significantly with precipitation intensity, and then remained largely stable. On the annual time scale, the slope and intercept of the LMWL in the cold half of the year were smaller than those in the warm half of the year. The possible reasons for the differences in the LMWL on different time scales are the combined effects of seasonal differences in precipitation intensity and water vapor sources.

Abstract. Seasonal variation and influencing factors of water isotopes were investigated in the Xiangjiang River basin, located in the East Asian monsoon region. This involved comprehensive sampling of precipitation and river water, as well as observing hydrometeorological factors spanning 13 hydrological years from January 2010 to December 2022. Key findings are as follows: River water δ2H (δ2HR) exhibited significant seasonal variation, with the most positive and negative δ2HR occurring in the spring flood period and summer drought, respectively, and generally aligned with those observed in precipitation. The correlations of the δ2HR with the corresponding hydrometeorological factors were generally weak and the reasons can be attributed to the seasonality of precipitation isotopes and mixing of various water bodies within the basin, but the changes in the runoff (ΔR) and δ2HR (Δδ2HR) between two contiguous samplings showed significant responses to the corresponding accumulated precipitation and evaporation. These results underscore the potential of Δδ2HR as a variable that reflects the seasonal variations in local environments, valuable for paleoclimate reconstruction. Prolonged rainless intervals with high evaporation rates in 2013 and 2022, as well as significant precipitation events in major flood periods in 2011 and 2017, notably had a significant impact on the δ2HR and runoff discharge. The most positive δ2HR values were primarily influenced by the precipitation input with the most enriched isotopes in the spring flood period, while the moderately isotope-depleted precipitation during limited basin wetness conditions led to the most negative δ2HR, thus caution is advised when interpreting extreme isotopic signals in river water. The spatial correlation analysis between water isotopes and hydrometeorological factors at the observing site and in the surrounding regions supported the representation of the Changsha site in the Xiangjiang River basin. Overall, these findings provide insights into the seasonal variation and influencing factors of δ2HR in the study area, shedding light on the complex dynamics of river water isotopes under different hydrometeorological conditions.

Stable isotopes respond sensitively to environmental evolution and record it in different geological archives due to fractionation. The stable isotope composition of water ( δO and δD) has been widely applied in fields including hydrometeorology, weather diagnosis, and palaeoclimate reconstruction. In recent years, the stable oxygen isotope of precipitation ( δO ) in southern China are valuable proxies of environmental variables, however, their interpretations of them have been controversial. Considering that the summer monsoon circulation brings about a large amount of precipitation, the temporal variation and range of the annual δO are consistent with those of the summer δO . Based on the observed and simulated data on oxygen isotope composition of precipitation, the linkage between summer precipitation ( P) and δO in the Dongting Lake Basin and their possible influencing factors are demonstrated. Meanwhile, the contribution of different factors is analyzed by using multiple linear stepwise regression. The results indicate that the temporal variation of summer δO is consistent with that of annual δO and the amount effect is identified in summer in the basin. Besides, the annual δO shows a significantly negative correlation with summer precipitation. It demonstrates that the value of stable isotopes in precipitation may be considered a proxy of summer precipitation in Dongting Lake Basin. Statistically, on an interannual timescale, the more south-westerly the Western Pacific Subtropical High (WPSH) extended, the higher the δO was and vice versa, indicating that there appeared circulation effect in the basin. In regression models based on the observed data in Changsha and the simulated data for the entire basin, the local summer precipitation always served as an important factor. It can be exemplified by comparing the local and upstream rainout along with water vapor flux transporting pathways in dry and wet summers. These results can potentially improve the reconstruction of paleoclimate in the East Asian monsoon region. Further study is needed to determine the contribution of local and large-scale factors to the oxygen isotope composition of precipitation.

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