Combining stable water isotopes and streamflow observations to model rainfall-runoff dynamics at 10-min resolution in a steep alpine catchment

We present a novel framework that leverages stable water isotopes measurements (δ¹⁸O) across multiple hydrological fluxes (snowpack, rainfall, several springs and stream) to constrain dominant runoff-generation processes during two summers in a snow-dominated headwater catchment in the western Swiss Alps (Vallon de Nant). To achieve this, a new flux partitioning routine has been developed to separate the contributions from (i) snowmelt, (ii) direct runoff, (iii) fast subsurface, and (iv) slow subsurface flow paths to streamflow. This routine has then been integrated into a simple hydrological model which combines the Geomorphological Instantaneous Unit Hydrograph and multiple subsurface reservoirs.

Results show that the model is able to simulate both streamflow and stream water isotopic composition at very high temporal resolution (10-minute intervals). By jointly calibrating the model with streamflow and streamflow isotopes during the summer season, we observe altered flux partitioning, with smaller event-water contribution to streamflow routed through direct runoff. We also find that incorporating stable water isotopes helps to refine model structure, to reduce parameter uncertainty, and to improve process attribution of short-timescale rainfall-runoff dynamics.