In spite of trying to understand processes in the same spatial domain, the catchment
hydrology and water quality scientific communities are relatively disconnected
and so are their respective models. This is emphasized by an inadequate
representation of transport processes, in both catchment-scale hydrological and
water quality models. While many hydrological models at the catchment scale
only account for pressure propagation and not for mass transfer, catchment scale
water quality models are typically limited by overly simplistic representations of
flow processes. With the objective of raising awareness for this issue and outlining
potential ways forward we provide a nontechnical overview of (1) the importance
of hydrology-controlled transport through catchment systems as the link
between hydrology and water quality; (2) the limitations of current generation
catchment-scale hydrological and water quality models; (3) the concept of transit
times as tools to quantify transport; and (4) the benefits of transit time based
formulations of solute transport for catchment-scale hydrological and water quality
models. There is emerging evidence that an explicit formulation of transport
processes, based on the concept of transit times has the potential to improve
the understanding of the integrated system dynamics of catchments and to provide
a stronger link between catchment-scale hydrological and water quality
models.