Relationship between sub-cloud secondary evaporation and stable isotopes in precipitation of Lanzhou and surrounding area

Based on the 420 samples of precipitation and related meteorological parameters obtained from the four sampling sites (Yongdeng, Gaolan, Lanzhou and Yuzhong) in Northwest China from April 2011 to February 2013, the influence of sub-cloud secondary evaporation effect on stable isotopes in precipitation was analyzed. Four main factors affecting the secondary evaporation were precipitation, air temperature, water vapor pressure, and relative humidity. The results showed that sub-cloud secondary evaporation had a significant effect on isotopes when the rainfall amount was small, but the correlation was not significant for snowfall or heavy rainfall. As the temperature increased, the secondary evaporation was enhanced. Water vapor pressure greatly impacted the sub-cloud secondary evaporation of the rain, but had less influence on the snow events. Relative humidity showed an influence on d-excess value, as well as the slope and intercept of the δD-δ18O correlation equation of light rainfall, but had a small impact when snow occurred. The estimated secondary evaporation rate was generally lower in winter and higher in summer, and spatially varied depending on locations. During the summer monsoon period (June to September), the secondary evaporation rate was estimated to be between 5.90% and 10.50% for each station with the mean value of 8.30%, and during the winter monsoon period (October to May), the rate was between 3.20% and 5.62%, with the average value of 4.54%.