Using high resolution isotope data and alternative calibration strategies for a tracer-aided runoff model in a nested catchment

Testing hydrological models over different spatio-temporal scales is important both for evaluating diagnostics and aiding process understanding. High-frequency (6hr) stable isotope sampling of rainfall and runoff was undertaken during 3 week periods in summer and winter within 12 months of daily sampling in a 3.2 km² catchment in the Scottish Highlands. This was used to calibrate and test a tracer-aided model to assess the: (1) information content of high resolution data; (2) effect of different calibration strategies on simulations and inferred processes; (3) model transferability to <1 km² sub-catchment. The 6-hourly data were successfully incorporated without loss of model performance, improving the temporal resolution of the modelling, and making it more relevant to the time dynamics of the isotope and hydrometric response. However, this added little new information due to old-water dominance and riparian mixing in this peatland catchment. Time variant results, from differential split sample testing, highlighted the importance of calibrating to a wide range of hydrological conditions. This also provided insights into the non-stationarity of catchment mixing processes, in relation to storage and water ages, which varied markedly depending on the calibration period. Application to the nested sub-catchment produced equivalent parameterisation and performance, highlighting similarity in dominant processes. The study highlighted the utility of high-resolution data in combination with tracer-aided models, applied at multiple spatial scales, as learning tools to enhance process understanding and evaluation of model behaviour across nonstationary conditions. This helps reveal more fully the catchment response in terms of the different mechanistic controls on both wave celerites and particle velocities.