Precipitation stable isotope composition, moisture sources, and controlling factors in Xi'an, Northwest China

Abstract

Seasonal and interannual variations in precipitation stable isotopes are crucial for clarifying complex moisture sources at the inland margins of the summer monsoon such as Northwest China. Stable hydrogen and oxygen isotope data (δD, δ18O) of totally 317 event-based precipitation samples from 2016 to 2020 in Xi'an show a consistent annual cycle in which, maximum δD and δ18O values occur in the pre-monsoon season (April–June), and minimum in the late monsoon to post-monsoon (November–March) seasons. Values of d-excess are lowest in the monsoon season. The local meteoric water line is δD = 7.5δ18O + 9.7. Precipitation δ18O in monthly time scale correlates weakly with relative humidity in the monsoon season, and with precipitation amount in the pre-monsoon season. Maps of potential water sources and wind fields, considered with monsoon intensity indices and the isotope data, are consistent with moisture sources as follows: during the monsoon, moisture advected by air flow mainly from the Bay of Bengal or recycled from land surfaces along this path; during the pre-monsoon, moisture advected in westerly circulation passing north of the Tibetan Plateau; and during the post-monsoon, recycled monsoon moisture advected from the south or southwest of Xi'an, and influenced by the intensity of the preceding South Asian Summer Monsoon (SASM). Moisture recycled from land surfaces is important in all seasons. The SASM index controlled the variation of mean seasonal δ18O, but not precipitation amount in the monsoon season. The westerly index (WI) controlled the precipitation amount in the pre-monsoon season and mean seasonal δ18O in the post-monsoon season. Seasonal change in moisture sources is the main reason of the seasonal variation of mean precipitation δ18O.