Abstract:

To assess the results of water stable isotopes fractionation simulated respectively by equilibrium model and kinetic model, four evaporation experiments were conducted under different atmospheric conditions. The results indicate that stable isotopes in residual water are gradually enriched along with the evaporation, there is a positive correlation between the enrichment rate and the evaporation rate. However, when precipitation happens, the residual water stable isotopes are diluted, as they are susceptible to the influence of relative humidity and the stable isotopes in atmospheric water vapor. Experimental water stable isotope fractionation rate is influenced by temperature and relative humidity, and certain positive correlation between the isotopic fractionation rate and the temperature changes appears. This phenomenon is contrary to the results described by Rayleigh fractionation model. There is a remarkable inverse relationship between the isotopic fractionation rate and relative humidity. Although the simulation results of equilibrium fractionation model have higher correlation coefficient with the measured results，as a whole，they fail to reflect the details of changes in stable water isotopes ratio with f in actual evaporation process, especially during the middle period of evaporation．In addition, the results of equilibrium simulation overestimate the degree of stable isotope fractionation. By contrast, kinetic fractionation model performs well in reproducing the variation of stable isotopes in water evaporation. It can capture the details of changes of the δ¹⁸O. The actual evaporation line slopes (3.855, 3.749, 4.097, 6.942) are low in summer and high in winter due to the influences of air temperature and relative humidity. Evaporation line slopes calculated from equilibrium fractionation model remain near 8, and their intercepts are all more than 10, close to the global meteoric water line, reflecting a poor fitting result. However, the evaporation line slopes (4.265, 3.433, 5.705, 5.833) of kinetic fractionation model are closer to those of the measurements, which can reflect the actual process of water evaporation. Variations of observed d-excess in residual water with residual water ratio f show a decrease trend in the four experiments, d value increases when rain events occur．And the decline speed of d-excess in summer is higher than that in winter, that is related to faster evaporation rate and stable isotope fractionation rates in summer. The d-excess of the equilibrium fractionation model varies with a constant value, and have lower correlation coefficient with the measured results, the root mean square error is large, and the simulation effect is poor, while that of the kinetic fractionation model shows similar results with measured d-excess both in magnitude and trend. It is concluded that the kinetic fractionation model would be more suitable to describe the water stable isotopes evaporation fractionation process under regional climatic conditions．

Key words: evaporation pan, evaporation fractionation, stable isotopes, equilibrium simulation, kinetic simulation