Stable Isotopes Reveal Water Vapor Sources of Precipitation over the Jiaolai Plain, Shandong Peninsula, China

A prerequisite for using isotopic techniques to study the regional water cycle of a mountainous area is to examine the stable isotopic composition of precipitation. These findings are of great significance for an in-depth understanding of water cycle processes. In this study, each precipitation event was sampled and used to investigate the characteristics of stable hydrogen and oxygen isotopes in precipitation over the Jiaolai Plain and its surrounding areas. NCEP/NCAR data was used for the wind speed and direction, relative humidity, and precipitable amount in the study area during the sampling period. The water vapor sources of the precipitation over the plain were revealed through a comparative analysis of seasonal variations in precipitation isotopes between Global Network of Isotopes in Precipitation stations located along different vapor transport paths. The results showed that the local meteoric water line was δ2H = 6.38 δ18O + 0.72, with a gradient of less than 8. This indicates that the precipitation process was affected by non-equilibrium evaporation occurring when the drops fell below the cloud base. Temperature and amount effects were observed in the δ18O of the precipitation, although the altitude effect was not significant. The water vapor source of the precipitation was predominantly controlled by the East Asian Monsoon from June to September, with the primary source being evaporation from the adjacent Pacific Ocean. The plain was controlled by the Westerlies from October through May, with the predominant vapor source being local evaporation. Water vapor from the polar region had a minimal impact. These findings can serve as the basis for studying surface water–groundwater–seawater transformations.