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.