Effect of below-cloud secondary evaporation on the stable isotopes in precipitation over the Yangtze River basin

Stable isotope compositions of hydrogen (δ²H) and oxygen (δ¹⁸O) for snow and rain were determined for a total of 443 GNIP (Global Network of Isotopes in Precipitation) samples collected from the Yangtze River basin.A correlation equation (δ²H=7.965δ¹⁸O+17.114) is obtained through linear regression between δ²H and δ¹⁸O values of snow samples.The maxmial slope and intercept values are shown in the snow equation.In contrast, correlation equations between δ²H and δ¹⁸O values for rain samples of four categories ranging from less than 10 mm to greater than 300 mm can result in prog ressively lower slope and intercept values with decreasing precipitation amount.The slope and intercept values in the four rain equations vary from 5.705 to 7.701 and -5.479‰ to 7.812‰, respectively.Analysis of the slope and intercept values of δ²H-δ¹⁸O correlation equations with amtospheric parameters such as temperature and water vapor pressure suggests that only the light rainfall events undergo secondary evaporation accompanied by isotope fractionation during raindrops descent from the cloud base to the ground.Since the light rainfall events can account for only 6.32% of the all precipitation events in the Yangtze River Basin Thus, the effect of below-cloud secondary evaporation on the stable isotopes in precipitation can only result in a slight decrease of slope and intercept values of the local meteoric water line.The study shows that the isotope analysis of individual precipitation samples can yields valuable information that cannot be obtained by the long-term weighted average samples.