Water sources and threshold behaviors of streamflow generation in a mountain headwater catchment

•A combined approach of HydroGeoSphere(HGS) modeling and isotopic tracing was proposed.•Contributions of multiple water sources to stream flow generation were identified.•The process chain of rainfall infiltration and return to surface is vital for runoff.•Rainfall thresholds for both surface/subsurface flow were quantified for first time. Numerous studies have analyzed the event-scale rainfall-runoff response of hillslopes and catchments. However, there are still knowledge gaps when it comes to understanding the spatiotemporal sources of headwater catchment runoff and the threshold behaviors of multiple runoff components. The aim of this study is to investigate the complex runoff generation mechanisms for 244 rainfall events in a humid headwater catchment of eastern China over five years (2011–2015). We employ a combined approach of isotopic tracing (δ2H and δ18O) and fully-integrated surface/subsurface flow modeling (HydroGeoSphere) to identify streamflow compositions and how the process chain of “rainfall infiltration, mixing with soil water, and subsequent exfiltration to the hillslope surface” impacts streamflow generation in headwater catchments. Our results show that the streamflow consists of event and pre-event water. Event water is composed of direct runoff from rainfall (2 % in the total streamflow) and indirect runoff from infiltrated rainfall (24 %). Pre-event water comprises the exfiltrated shallow soil water of indirect runoff (37 %) and subsurface flow (37 %). The study highlights the significant role of indirect runoff, comprising infiltrated rainfall and pre-event shallow soil water, in streamflow generation, reaching a peak proportion of 81.4 % during rainfall events. In addition, we quantify the threshold behaviors (including the generation threshold, and critical threshold from increasing to decreasing state) for surface and subsurface flow, based on the analysis of the non-linear rainfall-runoff relationships observed in the catchment. Results show that rainfall amount of 33 mm and rainfall intensity of 9 mm/d are generation thresholds for direct runoff and critical thresholds for proportion changes of exfiltrated shallow soil water. Critical thresholds of 55 mm for rainfall amount and 12 mm/d for rainfall intensity are identified for proportion changes of infiltrated rainfall and total indirect runoff. Our findings highlight that the combined analysis of hydrological modeling and isotopic tracing provides an effective solution for qu