Using hydrogen and oxygen stable isotopes to estimate soil water evaporation loss under continuous evaporation conditions

Abstract

AbstractThe 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 the isotopic fractionation of soil water in the surface layer 0–30 cm was dominated by a gaseous‐dominated transport process, and that both the δ18O and δD values and evaporative intensity decreased with increasing 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 greater accuracy than that provided by δD. The relative errors in the evaporation loss calculated based on δ18O and δD 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.