Assessing the added value of high-resolution isotope tracer data in rainfall-runoff modelling

Abstract. A study has been undertaken in a small agricultural catchment in east Scotland with the objective of assessing the value of high-resolution isotope data for integration within hydrological models. Daily samples of precipitation and stream flow, collected over a year, have been analyzed for deuterium content using new laser spectroscopy technology. This study set out to evaluate whether the utility of such high-resolution isotope data outweighs the associated uncertainty, as well as uncertainty in discharge measurements. A multi-criteria approach was used to evaluate the simulation of two independent objective functions – stream flow and stream deuterium – against model and data errors. Particular emphasis was placed on different input and output sampling resolutions. The data indicate that moderate isotopic variability in the stream response between rainfall events, which is not captured using weekly tracer data, has little influence on model performance. In contrast, the input resolution of precipitation deuterium concentrations, which were much more variable, proved to be crucial. A low-parameterized Catchment Isotope Model (CIM) was developed and adequately reflects flow dynamics and deuterium peaks, but a simple assumption of ''good-mixing'' in the isotope module is not able to fully reproduce the daily stream deuterium dynamic. Multi-criteria evaluation, however, significantly constrained acceptable behavioural parameter sets and therefore reduced the model's degree of freedom. This approach has provided further assessment of the value of tracers in hydrological modelling, demonstrating their usefulness in terms of model conceptualization, development and calibration, which outweighs the additional parameter and measurement uncertainty.