ABSTRACT
The use of hydrogen and oxygen
stable isotopes in estimating soil water evaporation loss under
continuous-evaporation conditions is crucial for gaining insight into
soil water movement processes under different conditions. In this study,
via high-frequency meteorological monitoring and continuous soil water
measurements, we investigated the variation of hydrogen and oxygen
stable isotopes and soil water fluxes with soil depth and time for soil
water at different depths under continuous evaporation conditions. The
precipitation isotope δrain and soil water flux changes
were determined using the Craig–Gordon model.
It was shown that a
gaseous-dominated transport process dominated the isotopic fractionation
of soil water in the surface layer 0-30 cm, and that both the
δ18O and δ2H values, and the
evaporative intensity decreased with soil depth. In terms of time
dynamics, the evaporation loss of soil water varies continuously with
seasons and is the highest during summer. The use of
δ18O to quantify the soil water evaporation loss
provides a greater accuracy than that provided by δ2H.
The relative errors in the evaporation loss calculated based on
δ18O and
δ2H were 13% and 34%, respectively. A sensitivity
analysis of each parameter indicated that the relative error calculated
by the model is primarily determined by temperature and relative
humidity uncertainty. The sensitivity analysis reveals the critical
evaporation intensity of soil water at various depths from unsteady to
steady state evaporation. When the relative humidity changes by 1%, the
evaporation loss fraction changes from 0.001 to 0.034. The results of
this study are important for quantifying the soil water resources in
arid and semi-arid areas without precipitation using stable isotopes of
hydrogen and oxygen.