Examining the impacts of precipitation isotope input (δ¹⁸O_ppt) on distributed, tracer-aided hydrological modelling

Abstract. Tracer-aided hydrological models are becoming increasingly popular tools as they assist with process understanding and source separation, which facilitates model calibration and diagnosis of model uncertainty (Tetzlaff et al., 2015; Klaus and McDonnell, 2013). Data availability in high-latitude regions, however, proves to be a major challenge associated with this type of application (Tetzlaff et al., 2015). Models require a time series of isotopes in precipitation (δ¹⁸O_ppt) to drive simulations, and throughout much of the world – particularly in sparsely populated high-latitude regions – these data are not widely available. Here we investigate the impact that choice of precipitation isotope product (δ¹⁸O_ppt) has on simulations of streamflow, δ¹⁸O in streamflow (δ¹⁸O_SF), resulting hydrograph separations, and model parameters. In a high-latitude, data-sparse, seasonal basin (Fort Simpson, NWT, Canada), we assess three precipitation isotope products of different spatial and temporal resolutions (i.e. semi-annual static, seasonal KPN43, and daily bias-corrected REMOiso), and apply them to force the isoWATFLOOD tracer-aided hydrologic model. Total simulated streamflow is not significantly impacted by choice of δ¹⁸O_ppt product; however, simulated isotopes in streamflow (δ¹⁸O_SF) and the internal apportionment of water (driven by model parameterization) are impacted. The highest-resolution product (REMOiso) was distinct from the two lower-resolution products (KPN43 and static), but could not be verified as correct due to a lack of daily δ¹⁸O_ppt observations. The resolution of δ¹⁸O_ppt impacts model parameterization and seasonal hydrograph separations, producing notable differences among simulations following large snowmelt and rainfall events when event compositions differ significantly from δ¹⁸O_SF. Capturing and preserving the spatial variability in δ¹⁸O_ppt using distributed tracer-aided models is important because this variability impacts model parameterization. We achieve an understanding of tracer-aided modelling and its application in high-latitude regions with limited δ¹⁸O_ppt observations, and the value such models have in defining modelling uncertainty. In this study, application of a tracer-aided model is able to identify simulations with improved internal process representation, reinforcing the fact that tracer-aided modelling approaches assist with resolving hydrograph component contributions and work towards diagnosing equifinality.