Groundwater dominance in streamflow generation: Insights from stable isotopes and tritium tracing in a semi-humid monsoonal catchment

The study was carried out in the Xitaizi Experimental Watershed (XEW), a semi-humid, monsoonal, forested headwater catchment in North China. Over a three-year period (2021–2023) spanning diverse hydroclimatic conditions, stable isotopes (δD, δ¹⁸O) and tritium (³H) were used to investigate streamflow sources, flow pathways, mixing dynamics, and water age. Streamflow composition was quantified using an End-Member Mixing Model (EMMM), and ages were estimated using Lumped Parameter Models (LPM). Old water was found to dominate streamflow, contributing 70.5–92.4 % at seasonal scales and 65.4–99.4 % at event scales. A strong parabolic relationship (R² = 0.94) between the old-water fraction and the hillslope–channel hydraulic gradient highlights distinct generation mechanisms under different hydrological regimes. Tritium-derived streamwater ages (mean > 10 years) matched groundwater and spring ages and were substantially older than soil water (< 10 years), indicating persistent and substantial groundwater contributions. Unconfined groundwater provided a steady baseflow source across conditions, while storms triggered preferential soil-water release and saturated overland flow in riparian zones. During prolonged drought, increased input from confined groundwater led to streamwater ages that exceeded those of bedrock groundwater. Study limitations include missing soil-water samples in dry years, storm-triggered rainfall sampling that may smooth short isotopic variability, and a simplified two-end member EMMM. These constrain source resolution and age precision, highlighting the need for additional tracers, higher-frequency sampling, and time-variant modelling. • Old water dominates streamflow across all seasons and events. • Hillslope-channel gradient controls old water fraction parabolically. • Streamwater mean age > 10 years, matching groundwater and spring. • Drought increases confined groundwater contribution to streamflow.