Consistent modelling of transport processes and travel times – coupling hydrologic processes with StorAge Selection functions

Understanding the transport processes and travel times of pollutants such as nitrate or pesticides in the subsurface is crucial for an effective management of drinking water resources. Transport processes and hydrologic processes, like infiltration, percolation, root water uptake or runoff generation processes, are inherently linked to each other. In order to account for this link, we couple the process-based hydrologic model RoGeR (including infiltration in the soil matrix, macropores and cracks) with StorAge Selection (SAS) functions. We assign to each hydrological process a specific SAS function (e.g. beta-type distribution or power distribution). To represent different transport mechanisms, we combined a specific set of SAS functions into four transport model structures: complete-mixing, piston flow, advection-dispersion and advection-dispersion with time-variant parameters. In this contribution, we quantify and illustrate the results of our modelling experiments at the Rietholzbach lysimeter, Switzerland. We compare our simulations to the measured hydrologic variables (percolation and evapotranspiration fluxes and soil water storage dynamics) and the measured water stable isotope signal (¹⁸O) in the lysimeter seepage for a period of ten years (1997-2007). An additional artificial bromide tracer experiment was used to benchmark the models. Additionally, we carried out a sensitivity analysis and provide Sobol’ indices. Our results show that the advection-dispersion transport model with time-variant parameters produces the best results. And thus, advective-dispersive transport processes play a dominant role at Rietholzbach lysimeter. Our modelling approach provides the capability to test hypotheses of different transport mechanisms and to improve process understanding and predictions of transport processes. Overall, the combined model allows a very effective simulation of combined flux and transport processes at various temporal and spatial scales.