Observational Study Of Evaporative Isotope Kinetic Fractionation Coefficients In Inland Water Bodies

Hydrogen and oxygen stable isotopes can provide unique tracer information for hydrological cycle.Evaporation process over inland water is an important part of hydrological cycle.Accurate quantification of the stable isotope fractionation effect during evaporation process is critical to the application of stable isotope method in hydrological cycle research.The stable isotope fractionation of water evaporation process under natural conditions is weakened by atmospheric turbulence,but the influence extent and controlling factors are not directly verified based on field experiments,which limits the accuracy of the stable isotope method in tracing inland hydrological cycle.This study centers on the isotope fractionation mechanism during evaporation over inland water bodies and aims to identify the degree of stable isotope fractionation weakened by the atmospheric turbulence.The water budgets and their isotopic compositions were observed under natural condition over three kinds of typical inland water bodies:Lake Taihu,fishpond and evaporation pans that can represent large water bodies,small water bodies and artificial water,respectively.The optimal value and uncertainty of the kinetic fractionation coefficient?_kwere calculated based on the combination of Craig-Gordon model and the isotope mass balance(IMB)model,and Unified Craig-Gordon(UCG)model.The IMB model and local evaporation line model were combined to contrast the tracing effect of D and ¹⁸O isotopes for evaporation,and to test the applicability of traditional?_k parameterization.Finally,the relationship between?_k and environmental factors such as water body area,local evaporation line and free convection intensity were discussed,and a parameterization of?_k applicable to inland water bodies were proposed.The main results are as follows:(1)The optimal?_k values over Lake Taihu,fishpond,big and small evaporation pan were calculated.The results indicated that the optimal?_k for H₂¹⁸O over Lake Taihu was 8.26‰based on the combination of flux-gradient method and the CG model.The optimal?_k for H₂¹⁸O were10.2±4.9‰for fishpond,14.3±6.8‰for big evaporation pan and 7.0±3.1‰for small evaporation pan based on the combination of IMB model and CG model.Combining the observation data from literature and this study,the?_k values for H₂¹⁸O of different water bodies were calculated using UCG or IMB models,and the overall mean value was 9.74±2.60‰for all water bodies considered with the corresponding turbulence parameter n is 0.34.With this n value,the corresponding?_k for HDO is 8.50±2.29‰.(2)In previous studies,the?_k parameterization widely used in hydrological research was usually derived based on laboratory experiments or physical models.In this study,the performance of these two traditional parameterization and the optimized?_k above were further evaluated in terms of estimating evaporation.The results show that the optimized value of?_kperformed best,followed by the?_k algorithm derived from physical model which is more accurate than that from laboratory experiments.(3)In the study of kinetic fractionation coefficients,the tracing effect of two stable isotopes HDO and H₂¹⁸O are quite different.The?_k values for H₂¹⁸O are more sensitive than HDO.The reason is that the isotope fractionation process of HDO was mainly dominated by equilibrium fractionation and it is not sensitive to the variation of kinetic fractionation effect,so it is hard to derived the?_k for HDO using the same method as H₂¹⁸O.(4)The environmental factors influencing the kinetic fractionation coefficient?_k were investigated,and the?_k parameterization scheme for inland water was established.There is no significant correlation between?_k and local evaporation line slope or water body area.But there was a significant negative correlation between?_k and T_s–T_a(y=-3.63 x+17.67;r=-0.64,p<0.01),implying that the kinetic fractionation effect was weaker with increasing convective turbulence intensity.Therefore,a new parameterization scheme of?_k can be established based on the function above.The results of this study can provide scientific basis for isotopic kinetic fractionation of inland water evaporation process,and promote the study of hydrogen and oxygen stable isotopes in inland water cycle.