Modelling the Soil Evaporation Loss in Secondary Forests of the Subtropical Monsoon Region, Central South China

Under more frequent, extreme global drought events, the use of stable isotopes to quantify soil evaporation losses (SEL) is of great significance for understanding the water supply capacity from soil to plants. During March 2017–September 2019, we continuously monitored meteorological factors, soil temperature (ST) and humidity, and collected precipitation and soil water stable isotope data. The Craig-Gordon (C-G) and line-conditioned excess (lc-excess) coupled with the Rayleigh fractionation (RL) models were used to quantify SEL in subtropical secondary forests. The results showed: (1) the theoretical evaporation line (EL) slope negatively correlated with air temperature (AT). Water source isotopic values are 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 during March–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 better suitability of the C-G model in 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 linear fitting method 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 of the different fundamentals, the C-G model positively correlated with AT and negatively correlated with relative humidity (h), while the RL model showed the opposite trends, indicating different applicability. The SEL is influenced by soil thickness, atmospheric evaporation and soil water supply capacity. These findings support stable isotope application techniques for quantifying SEL and are crucial for analysis of soil water resources in subtropical secondary forests.