Experiments And Simulations Of Water Isotopes Fractionation Effect In The Open Water Evaporation

Evaporation is the beginning of water cycle,it is the origin of atmospheric water vapor as well.The stable isotopes fractionation during the evaporation process plays a major part in leading to the distinct difference of stable isotopes in the subsequent stages of water cycle.Taking advantage of the variations of stable isotopes during free surface evaporation,one can fully analyze the evaporation fractionation effects along with their influencing mechanisms.It also contributes to testifying the applicability of diverse fractionation theories by observing the stable isotopes fractionation of evaporation water under different atmospheric conditions.Based on the free surface water evaporation experiments and lake water observation experiments carried out in evaporation pan and Peach Lake in Changsha,respectively,this paper analyzed the variation patterns of stable isotopes in residual water in evaporation pan under different controlling conditions and their relationships with meteorological factors.Additionally,we investigate the variation characteristics of stable isotopes in lake water and their influencing factors.We also discussed the variations of stable isotopes in residual water in evaporation pan under equilibrium fractionation and kinetic fractionation conditions.At the same time,we simulated the variations of stable isotopes in lake water.The results show that,(1)The ratio of stable isotopes in residual water shows an increase trend along with the evaporation,but there exist discrepancies in the detailed changes in the evaporation pan under these two controlling conditions.When rain events occur,the stable isotopes in residual water fluctuate in the evaporation pan without precipitation recharge,while they are sharply depleted in the evaporation pan with precipitation recharge.There is a positive relationship between the residual water stable isotopic fractionation rate and air temperature,whereas it shows a negative relationship between the isotopic fractionation rate and relative humidity.The maximum correlation coefficient appears between isotopic fractionation rate and evaporation subtract precipitation amount.(2)In the time distributions,the seasonal variation of ?18OL was significant,with the maximum ?18OL occurred in the spring,and the minimum in the winter,respectively.The high values of d-excess in lake water(dL)were recorded in the period of May and June,and the low values in during the period from September-October.In the spatial distribution,the differences of ?18L among different depths and spatial sampling points of Peach Lake were not significant,indicating that the lake was basically in a state of homogeneous mixing.The equation of lake water evaporation line is ?2H = 7.11?18O+0.50,obviously deviates from the local meteoric water line in Changsha which is ?2H = 8.33?18O +15.91.Moreover,these two lines have the same seasonal variations.These phenomena reflect that the precipitation is the major recharge source of lake water.Meanwhile,the lake water has experienced evaporation to some extent.(3)The correlation between the variation of ?18OL(??18OL)and cumulated 5-day precipitation amount(Ps)was significant,but the ??18OL was weakly correlated with the cumulated 5-day evaporation(Es),and there was no significant correlation between the variation of dL(?dL)and Ps,but the ?dL was weakly correlated with Es when the Ps were more than or equal to 20.3 mm.Compared with the average Lake Evaporation Line(LEL),the slope and intercept of the LEL increased significantly with Ps more than 20.3 mm.Furthermore,when the Ps were less than 20.3 mm,the correlation between ??18OL and Ps was significantly decreased,but the correlation between ?18OL and Es was significantly increased.Meanwhile,both the relationships between the variation of dL,and Ps as well as Es were significantly improved.On an average,the slope and intercept of the LEL were significantly reduced.Furthermore,in spite of the quite good consistency between the groundwater and lake water stable isotopes,the groundwater may not be the direct cause of the stable isotopic variations in lake water.(4)The variation trend of stable isotopic ratio in the residual water simulated by the equilibrium fractionation model shows some similarity with measured results.However,the decreasing trend caused by the high relative humidity and the interference of atmospheric water vapor can't be revealed using equilibrium fractionation model.The kinetic fractionation model is able to capture the detailed changes in residual water stable isotopic ratio under different atmospheric conditions.By contrast,both the higher correlation coefficient and the lower root mean square error between the kinetic simulations and the measurements elucidate that the kinetic fractionation model is more capable of reproducing the variations of the stable isotopes during the evaporation process,and the evaporation line of residual water simulated by the kinetic fractionation model would be more suitable to display the actual evaporation process.(5)The effects of our model not only reproduced the whole variation trend of the lake water stable isotopes,but also rightly captured the detailed changes of them under synoptic time scales.The higher correlation coefficient between the ?18OL(?2HL)simulations and their measurements,the slope of the linear regression equation near 1,coupled with the lower root mean square error,all of them consistently support the validity of the model in simulating the variations of lake water stable isotopes.Besides,both the simulated evaporation line(?2H = 6.08?18O-4.07)and the d-excess are close to the measured values,these findings furtherly verified our conclusions.