Separating snow and ice melt using water stable isotopes and glacio-hydrological modelling: towards improving the application of isotope analyses in highly glacierized catchments

Abstract. Glacio-hydrological models are widely used for estimating current and future streamflow across spatial scales, utilizing various data sources, notably observed streamflow and snow and/or ice accumulation, as well as ablation observations. However, modelling highly glacierized catchments poses challenges due to data scarcity and complex spatio-temporal meteorological conditions, leading to input data uncertainty and potential misestimation of the contribution of snow and ice melt to streamflow. Some studies propose using water stable isotopes to estimate shares of rain, snow and ice in streamflow, yet the choice of the isotopic composition of these water sources significantly impacts results. This study presents a combined isotopic and glacio-hydrological model which provides catchment-integrated snow and ice melt isotopic compositions during an entire melting season. These isotopic compositions are then used to estimate the seasonal shares of snow and ice melt in streamflow for the Otemma catchment in the Swiss Alps. The model leverages available meteorological station data (air temperature, precipitation and radiation), ice mass balance data and snow cover maps to model and automatically calibrate the catchment-scale snow and ice mass balances. The isotopic module, building on prior work by Ala-Aho et al. (2017a), estimates seasonal isotopic compositions of precipitation, snow and ice. The runoff generation and transfer module relies on a combined routing and reservoir approach and is calibrated based on measured streamflow and isotopic data. Results reveal challenges in distinguishing snow and ice melt isotopic values in summer, rendering a reliable separation between the two sources difficult. The modelling of catchment-wide snowmelt isotopic composition proves challenging due to uncertainties in precipitation lapse rate, mass exchanges during rain-on-snow events and snow fractionation. The study delves into these processes and their impact on model results and suggests guidelines for future models. It concludes that water stable isotopes alone cannot reliably separate snow and ice melt shares for temperate alpine glaciers. However, combining isotopes with glacio-hydrological modelling enhances hydrologic parameter identifiability, in particular those related to runoff transfer to the stream, and improves mass balance estimations.