Assessment of streamflow components and hydrologic transit times using stable isotopes of oxygen and hydrogen in waters of a subtropical watershed in eastern China

Streamflow components (e.g., young water vs old water) and hydrological transit times play an important role in water resource and water quality management. We collected a four-year record of stable isotopes of oxygen and hydrogen (delta O-18 and delta H-2) in precipitation, groundwater and stream water for six catchments in the Yongan watershed of eastern China. The stable isotope records were used to identify spatio-temporal variations in the young water fraction (F-yw, defined as the proportion of the transit-time distribution younger than a threshold age) and mean transit time (MIT) based on sine-wave fitting and convolution integral methods, respectively. The F-yw ranged from 14 to 35% in the Yongan watershed. Cumulative transit time showed contrasting distributions, suggesting considerable heterogeneity and a complex interplay between catchment characteristics. Estimated MTTs ranged from 3.2 to 6.3 years and may be explained by catchment characteristics (e.g., elevation and topographic gradient). Observed spatial trends in MTTs likely result from contrasting contributions of different-aged subsurface water flows across the six catchments. The use of F-yw constraints in estimating MITs reduced uncertainty in some catchments, suggesting the potential benefits of combining multiple approaches (e.g., F-y(w)) to optimize the results of traditional calibration methods. The relatively low F-yw and long MTTs highlight the importance of groundwater contributions to streamflow generation and imply a considerable lag time in river water quantity and quality responses to catchment-scale water resource management. Coupling multiple metrics (e.g., F-yw and MTT) and isotope models enhances our understanding of watershed-scale hydrologic processes and hydrograph separation.