Influence of below-cloud secondary evaporation on stable isotope composition in precipitation and its relationship with meteorological factors in Shiyang River Basin, Northwest China

In arid regions, the process of precipitation from the cloud base to the ground is often affected by the below-cloud secondary evaporation, which changes the ratios of hydrogen and oxygen stable isotopes in falling raindrops. Based on stable isotopes of 670 precipitation samples and meteorological records at 11 stations in the Shiyang River Basin, and combined with the improved Stewart model, this study discussed the impact of different meteorological factors on below-cloud secondary evaporation. The result showed that the Local Meteoric Water Line (LMWL) of the Shiyang River Basin is δD = 7.70δ18O + 8.90 (r2 = 0.98, p < 0.01, n = 670). The secondary evaporation is stronger in spring and summer and weaker in autumn and winter, and it is stronger in the mid-downstream than that in the upstream. When temperature ranges from 10 °C to 20 °C, precipitation is less than 10 mm, and relative humidity is lower than 30%, the phenomenon of isotope dynamic fractionation caused by the secondary evaporation is more obvious. As temperature increases, relative humidity, precipitation amount, or raindrop diameter decreases, the linear relationship between the evaporation remaining ratios of raindrops (f) and the difference of deuterium excess from cloud base to ground (Δd) weakens gradually. Δd has a stronger response to the changes in relative humidity than that in temperature and precipitation intensity, and the impact of raindrop diameters more than 1.2 mm on Δd is very limited.