Seasonal Stable Isotope Variation in Precipitation on the Northwest Margin of the East Asian Monsoon Region: Insights from Air-Mass Transport Pathways and Implications for Water-Resource Sustainability

Understanding seasonal precipitation variability and air-mass transport is important for sustainable water-resource management in water-scarce monsoon-margin regions. Stable isotopes in precipitation provide useful tracers of these hydroclimatic processes. Based on 101 event-based precipitation samples collected in Shijiazhuang during 2015–2017, the local meteoric water line (LMWL) was determined as δD = 7.00δ18O + 4.11 (R2 = 0.93, p < 0.05). The isotope records revealed distinct seasonal variation. The δ18O–temperature relationship showed pronounced seasonality: δ18O was significantly and positively associated with temperature across all events and during the dry season, whereas no significant relationship was observed during the wet season. Six trajectory-defined categories were identified, with continental trajectories occurring more frequently than oceanward trajectories during the 2015–2017 observation period. Precipitation associated with different trajectory-defined categories showed contrasting δ18O and d-excess characteristics. The occurrence frequency of precipitation events and the fraction of observed precipitation amount associated with these trajectory categories varied with the seasonal evolution of the East Asian summer monsoon and were accompanied by corresponding seasonal variations in precipitation isotopes. HYSPLIT characterizes transport pathways rather than directly quantifying moisture-source contributions. These findings improve process-based understanding of seasonal precipitation inputs and regional hydrological-cycle sensitivity, providing hydroclimatic information relevant to water-resource sustainability and climate adaptation in water-scarce monsoon-margin regions.