Can time domain and source area tracers reduce uncertainty in rainfall‐runoff models in larger heterogeneous catchments?

We used both time domain (deuterium) and source area (alkalinity) tracers to reduce uncertainty in simple conceptual rainfall‐runoff models applied to a larger (749 km 2 ) heterogeneous catchment with both upland and lowland headwaters. Stepwise, tracer‐aided model development resulted in different model structures for the uplands and lowlands as representative elementary watersheds (REWs). These were differentiated by the parameterization of a nonlinear overland flow mechanism in the former, and the incorporation of high soil moisture storage capacity in the latter. Use of tracers and recession characteristics also helped to reduce parameter uncertainty and provided models that could simulate flows and tracer responses reasonably well over a full hydrological year. However, it was apparent that other processes (e.g., more complex mixing, fractionation, etc.) would need to be parameterized to explain the full variation in isotope dynamics. It is also evident that the information content of tracer data declines as the intensity of sampling decreases, particularly in the lowlands. The models of the REWs were coupled to provide plausible simulations of the up‐scaled flow and tracer response at the outfall of the 749 km 2 catchment, though the usefulness of source area tracers decreased markedly at this larger scale. Whereas the approach provides a step toward simple models that are likely to give the “right answer for the right reasons,” further improvements appear to require increased parameterization and/or higher‐resolution tracer data.