5. Conclusions
Our research provided a qualitative and quantitative mutual demonstration of soil water stable isotope dynamics at different depths under continuous evaporation conditions. The results showed that δ18O and δ2H decreased with increasing soil depth from 0-30 cm. Most soil water was transported in a gaseous state. Within 10 cm of the soil surface layer, evaporation had the greatest effect.
Furthermore, δ18O provided better results than δ2H for estimating evaporation losses overall. During the calculation periods extending from May 13, 2018 to July 12, 2018 and January 5, 2019 to April 20, 2019, the evaporation loss within a range of 5–30 cm from the surface layer was 1 mm and 14 mm, respectively. The relative error of the evaporation loss calculated based on δ18O and δ2H was 13% and 34%, respectively. The use of δ18O to quantify the soil evaporation loss resulted in an additional accuracy of 21% compared with that resulting from δ2H.
The control variables approach suggests that temperature and relative humidity constitute sensitive parameters of the model, and the critical intensity of evaporation for switching from unsteady to steady-state evaporation of soil water at different depths can be identified utilizing sensitivity analysis. Furthermore, changes in a single factor cannot be fully reflected in the model, and the parameters work synergistically.
The study focused primarily on the spatial and temporal heterogeneity of soil water isotope evaporation signals at different depths under conditions of continuous evaporation, and quantified the significant influence of evaporation losses. These results are valuable for understanding regional hydrological processes, and soil water resources planning in arid and semi-arid regions.