Toward representing the subsurface nitrate legacy through a coupled StorAge selection function and hydrological model (SWAT-SAS)

The transit time distribution (TTD) is a lumped method to characterize the diverse flow paths of a hydrological system, and the StorAge Selection function (SAS) is one of the time-variant forms, representing the link between storage and outflow. Although it provides age information about the water parcels and nutrient legacy, the spatial heterogeneity cannot be captured by this method. While the distributed physically-based hydrological models (PBHMs) can reflect the spatial heterogeneous in climate, land cover and management, its simplification of subsurface processes prevents it from representing the subsurface nutrient transport and sometimes fails to capture nutrient legacy dynamics of the landscape. We attempted to couple SAS functions into a PBHM (SWAT) for calculating the nitrate dynamics of aquifers. The results show that both SWAT-SAS and SWAT can reproduce the in-stream dynamics of streamflow and nitrate concentration for the Upper Selke catchment; the coupled model allows more flexibility of storage release schemes and provides water age information of the aquifers; even though both the models simulated comparable in-stream nitrate concentrations, the nitrate store remained in aquifers varied, which will have varying implications for pollution goal assessment and nutrient management.