Groundwater Dominance in Streamflow Generation of a Semi-Humid Headwater in North China: Insights from Isotope Analysis

Understanding water sources, flow paths, and mixing patterns in headwater catchments is essential for effective hydrological research, water resource management, and water-related disaster control. This study investigates the Xitaizi Experimental Watershed (XEW) in North China, a semi-humid monsoonal forested catchment, using stable isotopes (δD, δ¹⁸O) and tritium (³H) alongside End-Member Mixing Models (EMMM) and Lumped Parameter Models (LPM) over three hydrological years with varying hydroclimatic conditions. Results indicate that during storm events, old water constitutes 86.2% to 99.2% of streamflow, primarily influenced by rainfall amount and antecedent wetness. Tritium-based age estimations reveal groundwater ages of 14–20 years, soil water of 6–10 years, and river water of 6–9 years. The estimated active aquifer storage ranged 1.0–2.6 meters of water. The study highlights XEW’s substantial groundwater storage capacity, which consistently contributes to river flow under varying hydrological conditions, though preferential release of soil water occurs during storm event. These findings underscore the critical role of groundwater in sustaining streamflow and the necessity for careful water resource management. Additionally, the research emphasizes the importance of precise parameter selection in tracer-based modeling and calls for future high-resolution, long-term studies to further refine hydrological understanding.