Alpine headwater catchments often produce streamflow peaks anywhere from minutes to hours after a rainfall event, quickly mobilizing previously stored subsurface water. Understanding and describing this response to rainfall events and its variations via modeling remains challenging, despite the growing availability of data, including tracer data such as stable isotopes of water, to inform internal flow partitioning.</p> <p>Here, we present a novel flux partitioning routine integrated into an existing hydrological model based on the Geomorphological Instantaneous Unit Hydrograph and multiple subsurface reservoirs. The model structure is based on a detailed perceptual model derived from available field data, and it simulates streamflow magnitude and streamwater isotopic ratios at high temporal resolution (10-minute intervals) in a snow-dominated headwater catchment in the western Swiss Alps (Vallon de Nant) over two consecutive summers. By jointly calibrating the model with both streamflow quantity and streamwater isotopic ratios, we obtain a reliable representation of catchment-scale partitioning of streamflow between groundwater, snowmelt, and direct runoff. The simulations showed that despite the catchment’s high level of flashiness, with peak streamflow occurring within an hour of peak rainfall, most streamflow originates from subsurface flow pathways with limited contributions from direct runoff. Even during short periods of intense convective rainfall events, peak flows mostly comprise water released by subsurface storage, which are mobilized by rainfall.</p> <p>Our results show that incorporating stable water isotopes into a relatively simple catchment-scale model can refine model structure, reduce parameter uncertainty, and improve process attribution of short-timescale streamflow generation dynamics. We discuss the advantages and limitations of the proposed isotope-enabled modeling approach and its potential extension to other catchments and year-round modeling.