Analysis of Stable Hydrogen and Oxygen Isotope Characteristics and Vapor Sources of Event-based Precipitation in Chengdu

Abstract

Based on analysis of hydrogen and oxygen isotopes in 113 rainfall samples collected from September 2016 to October 2017 in Chengdu, which is a typical representative of humid areas affected by multiple moisture sources, the compositional characteristics of hydrogen and oxygen isotopes (2H, 18O, and 17O) and the water vapor sources of precipitation were analyzed. It was found that δD, δ18O, δ17O, d-excess, and 17O-excess in atmospheric event-based precipitation have significant seasonal variation. In the dry season they are high and in the wet season are low, reflecting the different moisture sources during two seasons (dry and wet). The slope and intercept of the Local Meteoric Water Line were small, indicating that the precipitation originated from sources with various stable isotope ratios and that raindrops were subject to secondary evaporation during their landing process. The Local Meteoric Water Line slope for the triple oxygen isotopes (δ'17O=0.5289δ'18O+0.0075) ranged between the slopes for seawater vapor and dry air, and the value of 17O-excess was far larger than that of seawater. This indicates that the Chengdu area lies in the path of marine air masses moving toward inland regions. The atmospheric precipitation mainly came from these marine air masses and the isotope had undergone serious enrichment in the process of reaching the area. The d values were close to the global average, and the extremely low value of d-excess in the dry season may be affected by artificial rainfall operations. In addition to the relative humidity of the water vapor source, 17O-excess is also affected by the upstream air mass convection; moreover, the 17O-excess of the precipitation was not affected by the meteorological factors over the whole study period, so the 17O-excess could be considered tracers of evaporative conditions at the vapor source in Chengdu. The precipitation 17O-excess in different seasons provides additional information to better understand the precipitation formation processes in Chengdu.