Under the more frequent and extreme global drought events, utilizing stable isotopes to quantify soil evaporation losses ( SEL) is of great significance for understanding the water supply capacity from soil to plants. From March 2017 to September 2019, we continuously monitored meteorological factors, soil temperature and humidity, and collected precipitation and soil water stable isotope data. Used the Craig-Gordon (C-G) model and the line-conditioned excess (lc-excess) couple with Rayleigh fractionation (RL) model to quantify SEL in subtropical secondary forests. The results showed: (1) The theoretical Evaporation Line (EL) slope correlated negatively with air temperature ( AT). Water source isotopic values were more positive in autumn and more negative in spring. The aridity index ( AI) and soil evaporation loss ratio ( f) from both models indicated drier conditions from March to September 2018 compared to 2017 and 2019; (2) Comparative analysis showed the C-G model agreed more closely with measured evapotranspiration ( ET0) and water surface evaporation ( E) than the RL model, indicating its better suitability for the study region; (3) Because the “inverse temperature effect” of the precipitation isotopes, the linear fitting method was not suitable for determining the water source in spring, summer, autumn, and on the annual scale, while the EL slope obtained by the fitted slope was consistent with the basic principle of soil evaporation in winter. Thus, the theoretical method was more suitable for determining the EL slope in such regions; (4) because the different fundamentals, the C-G model was positively correlated with air temperature and negatively with relative humidity ( h), while the RL model showed the opposite, indicating different applicability. Meanwhile, SEL is influenced by soil thickness, atmospheric evaporation, and soil water supply capacities. These findings support using stable isotope techniques to quantify SEL and are important for analyzing soil water resources in subtropical secondary forests.

Stable isotopes respond sensitively to environmental evolution and record it in different geological archives due to fractionation. The stable isotope composition of water ( δ 18O and δ 2D) has been widely applied in fields including hydrometeorology, weather diagnosis, and palaeoclimate reconstruction. In recent years, the stable oxygen isotope of precipitation ( δ 18O p) 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 δ 18O p are consistent with those of the summer δ 18O p. Based on the observed and simulated data on oxygen isotope composition of precipitation, the linkage between summer precipitation ( P) and δ 18O p 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 δ 18O p is consistent with that of annual δ 18O p and the amount effect is identified in summer in the basin. Besides, the annual δ 18O p 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 δ 18O p 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.