ISOTOPIC CHARACTERISTICS IN HUMID AREAS AFFECTED BY MULTIPLE MOISTURE SOURCES: A CASE STUDY IN CHENGDU, CHINA

Based on 113 rainfall samples collected from September 2016 to October 2017 in Chengdu, the compositional characteristics of hydrogen and oxygen isotopes (²H, ¹⁸O and ¹⁷O) and the moisture sources of precipitation are analyzed. The seasonal variations in δD, δ¹⁸O, δ¹⁷O, d-excess and ¹⁷O-excess of atmospheric precipitation suggest that the moisture sources are different during two seasons (dry and wet). The slope and intercept of the local meteoric water line (δD=7.83δ¹⁸O+8.79) are small, indicating that raindrops have experienced secondary evaporation during the landing process. The slope of the local meteoric water line for the triple oxygen isotopes (δ'¹⁷O=0.529δ'¹⁸O+0.0075) ranges between the slopes for sea water vapor and dry air, and the value of ¹⁷O-excess is much larger than that of seawater, which means that the Chengdu area lies in the path of marine air masses migrating toward inland arid regions. Atmospheric precipitation mainly derives from marine air masses, and isotopes experience serious enrichment during the migration process. The below-cloud secondary evaporation in Chengdu is mainly affected by the relative humidity (RH) and surface temperature (ST), while the influence of precipitation amount (PA) is not obvious. The variations in δD, δ¹⁸O, δ¹⁷O, d-excess and ¹⁷O-excess can be properly explained by the airflow field given by back-trajectory model, which demonstrates that monsoon activity is the major driving force for isotopic variations.