Using tracers to upscale flow path understanding in mesoscale mountainous catchments: two examples from Scotland

Natural geochemical tracers were monitored over a hydrological year in the Feshie (231 km2) and Feugh (233 km2) catchments in the Cairngorm Mountains, Scotland. The monitoring sought to assess the utility of tracers in upscaling flow path understanding in mesoscale catchments. A spatially and temporally nested sampling approach was adopted involving hydrochemical monitoring of sub-catchments ranging from 1 to ≈100 km2 in area. This allowed chemically-based hydrograph separations to be made and faciliated catchment-wide prediction of stream chemistry at a high and low flows. Differences in catchment geology were the main controls on baseflow chemistry, which was spatially variable in both catchments. However, acidic, organic soils produced the majority of storm runoff at all scales monitored, though its contribution was determined by the soil distributions of particular sub-catchments. Given the practical difficulties associated with comprehensive hydrometric monitoring at large spatial scales, it is argued that focused tracer studies can provide both an invaluable insight into the hydrological functioning of mesoscale catchments and a useful additional method for evaluating the structure and performance of distributed hydrological models. In addition, such tracer data provides important information on hydrological pathways that can aid catchment management.