Long-term spatiotemporal assessment of stable isotopic composition of precipitation across South Korea (2019–2023)

Mainland South Korea is a humid temperate peninsula characterized by strong seasonal climatic contrasts, complex topography, and a pronounced monsoon influence. The region comprises four major river basins (Han, Geum, Nakdong, and Yeongsan–Seomjin) and is influenced by both maritime and continental air masses. Precipitation is highly seasonal and strongly influenced by stationary fronts and typhoons, with over half of the annual precipitation occurring during the summer monsoon. Spatiotemporal trends in the stable isotopic compositions (δ¹⁸O and δD) of precipitation were investigated. A total of 617 samples collected from 24 monitoring stations between July 2019 and December 2023 were used in this study. Local meteoric water lines (LMWLs) were established for the entire dataset and for each season. Amount-weighted mean isotopic values were calculated to evaluate controlling factors. Strong seasonal controls on isotopic composition were observed. Summer precipitation follows the Global Meteoric Water Line, indicating near-equilibrium condensation under humid conditions. In contrast, winter precipitation shows lower LMWL slopes and higher intercepts, reflecting non-equilibrium processes and elevated deuterium excess associated with dry continental air masses and/or continental recycling over the Yellow Sea. Distribution patterns of δ values reveal continental, latitudinal, longitudinal, and altitudinal effects, except at three coastal and/or isolated stations (i.e., Gs, Kr, and Ph). This study provides the first long-term isotopic baseline for mainland South Korea, supporting future isotope-based hydrological studies, including groundwater recharge and water source identification.