Advancing tracer-aided rainfall-runoff modelling: A review of progress, problems and unrealised potential

Using conservative tracers to aid conceptual rainfall-runoff modelling has gained momentum over the past decade. Tracer data have been invaluable in providing rich insights into runoff sources, flow paths and water age that cannot be established by simple rainfall-runoff dynamics alone. Accordingly, this has provided a focus for fertile dialogue between field hydrologists and modellers in joint efforts to understand catchment function and incorporate this in runoff models. Central to this has been the utility of tracers in establishing the differences between the timescales of the celerity of the rainfall-runoff response and the timescales of the pore velocity of water. The literature now has numerous examples of using tracers to aid modelling as a learning framework. Despite this progress, utilization of tracer-aided models and exploitation of their evident advantages by the wider modelling community have been slow. This in part reflects lack of suitable data sets at many sites and the fact that studies to date have highlighted various problems and challenges when trying to integrate tracers into rainfall-runoff models (e.g. increased parameterisation). Nevertheless, interest in tracer-aided modelling has continued to build as there have been marked improvements in the reliability and economics of field and laboratory methods for collecting spatially distributed and high temporal resolution tracer data sets. Consequently, we stand on the threshold of unprecedented advances in applications of this area. Here, we critically evaluate the progress to date and assess the challenges that remain. The key current research frontiers with the greatest potential for rapid advancement are as follows: (1) to go beyond hydrograph simulation alone and build more realistic models of catchment functioning based on tracer data, (2) investigations into the nonlinear, threshold-type, non-stationary and hysteresis-driven nature of how catchments process water and solutes, (3) detailed eco-hydrological studies of connectivity patterns and the role of vegetation on water partitioning and (4) the assessment of anthropogenic influences on the catchment hydrological cycle.