Insights into spatiotemporal dynamics of riverine water age and its controlling factors in five contrasting subtropical headwater catchments, South-Central China using stable water isotopes

Abstract. Understanding river water age and its controlling factors are fundamental for comprehending catchment hydrological and biogeochemical processes. However, how do landscape characteristics and climate properties control the spatiotemporal heterogeneity of riverine water age remain to be further clarified in subtropical headwater catchments. This study used stable isotopic ratios (δD and δ¹⁸O) from five contrasting headwater catchments. The study explored time-variable young (F_yw) and new (F_new) water fraction among five contrasting headwater catchments from the upper reaches of Xiu River, located within the Poyang Lake catchment of South-Central China using stable isotopes (δD and δ¹⁸O) from 2021 to 2023. The isotopic compositions of precipitation exhibited greater fluctuations than those of river water from five sub-catchments. The lower slopes (3.78 to 6.63) and intercepts (-13.12 to 2.65) of linear regression correlations between δD and δ¹⁸O were observed in river water compared to the global and local meteoric water lines, indicating significant evaporation effects on river water. The young water fraction (F_yw) showed considerable spatial variability ranging from 0.07 to 0.21 among five sub-catchments, suggesting the dominant recharge of groundwater to the river. The pronounced temporal variations of F_yw highlighted its susceptibility to short-term hydroclimatic change. Random forest models revealed that precipitation (25.48±5.41 %) and potential evapotranspiration (27.84±6.62 %) were the primary drivers for young and new water generation. Furthermore, F_yw was significantly influenced by upstream inflows (24.21±0.71 %), whereas F_new was more susceptible to the influence of percentage of forest (22.63 %) and cropland (29.85 %). Shapley Additive Explanations reveal a significant negative correlation between river area and F_yw, and a significant positive correlation between agricultural area proportion and F_new. Combined with the dynamic variations in the F_yw and F_new, these results indicated that the regulatory function of riparian zones played a crucial role in young water generation, while land use changes significantly altered the process of new water generation. Our findings suggest that intensified evapotranspiration and increased precipitation will significantly impact the generation of riverine young and new water in the context of global warming and land use type changes.