Towards minimal calibration of process-based rainfall runoff models in mountainous watersheds: integrating tracer data into geomorphic instantaneous unit hydrographs

The Geomorphological Instantaneous Unit Hydrograph (GIUH) approach was applied for rainfall-runoff modelling in a 31 km2 montane catchment in the Scottish Highlands. Modelling was based on flow path length distributions derived from a Digital Terrain Model (DTM). The model was applied using a single landscape unit response based on the DTM, and a two-landscape unit response, which included saturated areas. This was to examine whether process information would enhance the model performance. The model was applied with limited calibration to produce parameter sets, which could be applied to a spectrum of contrasting events. Alkalinity was used as a conservative tracer to provide an additional objective measure ("soft" data) of model performance. The models captured the hydrological response of the catchment reasonably well and facilitated feasible predictions of alkalinity. The single landscape unit approach produced the best individual model performance statistics, probably because of limitations embedded within the GIUH approach. Nevertheless, the study suggested that GIUHs have considerable potential for prediction of hydrological response in ungauged montane basins.