Isotopic characteristics and migration of water vapor in the vadose zone of Mu Us Sandy Land

Vapor-water exchange among atmosphere, soil and groundwater plays an important role in the surface ecological restoration in the vadose zones of arid and semiarid regions. However, it is limited to in-situ monitoring of water vapor flux, due to the complex and uncertain process. The simulation of water vapor cannot be fully verified by the measurement. It is still lacking in the vapor-water migration at different spatial and temporal scales. This study aims to clarify the spatiotemporal variation and migration of vapor-water exchange in the vadose zone of Mu Us Sandy Land by in-situ vapor-water monitoring, isotope tracer and numerical analysis. The results showed that the δD_L and δ¹⁸O_L of precipitation were enriched in spring and summer, but depleted in autumn and winter, due to the atmospheric water vapor source and local circulation. Soil evaporation was enriched in the oxygen isotope (δ¹⁸O_a) of surface soil vapor-water in summer more than that of soil freezing in winter. Moreover, there was a significant positive correlation (P<0.01) in the δ¹⁸O profile of the soil vapor-water and liquid water at different depths. The reason was that the evaporation intensity source and migration mode of soil vapor-water varied greatly in seasons. The δ¹⁸O values of vapor water in spring and summer showed an extremely significant linear positive correlation with the liquid water (P<0.01), while there was no significant correlation in winter (P=0.12). Moreover, there was a positive linear relationship between water vapor flux and δ¹⁸O_a. Furthermore, the δ¹⁸O_a was enriched in the surface layer at the large water vapor flux, including both downward and upward migration. The surface δ¹⁸O_a was enriched in the period of soil freezing in winter, particularly with the decrease of water vapor density (November-March). But the surface layer δ¹⁸O_a increased in summer (May-October) as well with the increase of surface water vapor density. Driven by the gradient of soil temperature in the vadose zone, the replenishment relationship of water vapor throughout the profile varied greatly at different periods. The water vapor of shallow soil was the supply source of deep vapor-water in summer, while the recharge was received from the deep layer in winter. But there were the temperature convergence and divergence zero flux planes in spring and autumn, respectively. The recharge characteristics caused the complicated relationship of vapor recharge in the profile. The soil δ¹⁸O_a was controlled by the water vapor migration, atmospheric evaporation and soil freeze-thaw in the vadose zone. In winter, the reduced evaporation and upward transport of water vapor can concurrently cause to enrich the surface δ¹⁸O_a, thus resulting in a decrease in the correlation between soil water vapor δ¹⁸O_a and liquid water δ¹⁸O_L. While in summer, the dominant effect can be from the diurnal evaporation and condensation cycle of soil water. The findings can provide a scientific basis to clarify the migration of soil water vapor in the water circulation