Exploring Summer Precipitation Stable Isotopes in the Monsoon Border Zone: Environmental Drivers and Moisture Source Tracing

Abstract

Precipitation processes in the monsoon border zone (MBZ) of China remain unclear because of the complex hydro-climatic interactions. This study analyzes the temporal variation of δ²H and δ¹⁸O in precipitation and its influencing factors based on samples of precipitation events in Lake Dali during summer from 2018 to 2020. The results showed significant monthly isotopic variability, characterised by lower δ²H and δ¹⁸O values in July with averages of −103.62‰ and −14.87‰, respectively. This clear isotopic variability is largely related to the East Asian summer monsoon (EASM) activity during the prevailing summer monsoon season and hydrometeorological processes. The isotopic composition of precipitation is mainly determined by the amount of precipitation and relative humidity, but not related to temperature. The results of backward trajectory modelling show that precipitation primarily originates from northern China, inland Xinjiang, and the western Pacific with different monthly contributions. The increase in δ²H and δ¹⁸O isotopes from June to August indicates sub-cloud evaporation. The mean sub-cloud evaporation rate was 3.5% during the summer. Sub-cloud evaporation is associated with relative humidity and precipitation. Recycled moisture accounts for 12% of local precipitation and 2.5%–31.6% of total monthly precipitation. After considering the contribution of evaporation, the estimated average recycling ratio increased from 12% to 24%, suggesting the important role of evaporation processes in lakes in the formation of precipitation.