Agriculture disturbs the biogeochemical cycles of major elements, which alters the elemental stoichiometry of surface stream waters, with potential impacts on their ecosystems. However, models of catchment hydrology and water quality remain relatively disconnected, even though the observation that dissolved organic carbon (DOC) and nitrate (NO₃⁻) have opposite spatial and temporal patterns seems relevant for improving our representation of hydrological transport pathways within catchments. We tested the ability of a parsimonious model to simultaneously reproduce intra-annual dynamics of stream flow, DOC and NO₃⁻ concentrations using 15 years of daily data from a small headwater agricultural catchment (AgrHyS observatory). The model consists of an unsaturated reservoir, a slow reservoir representing the groundwater and a fast reservoir representing the riparian zone and preferential flow paths. The sources of DOC and NO₃⁻ are assumed to behave as infinite pools with a fixed concentration in each reservoir that contributes to the stream. Stream concentrations thus result from simple mixing of slow and fast reservoir contributions. The model simultaneously reproduced annual and storm-event dynamics of discharge, DOC and NO₃⁻ concentrations in the stream, with calibration KGE scores of 0.77, 0.64 and 0.58 respectively, and validation KGE scores of 0.72, 0.58 and 0.43 respectively. These results suggest that the dynamics of these concentrations can be explained by hydrological transport processes and thus by temporally variable contributions from slow (NO₃⁻ rich and DOC poor) and fast reservoirs (DOC rich and NO₃⁻ poor), with a poor representation of the biogeochemical transformations. Unexpectedly, using the concentration time series to calibrate the model increased uncertainty in the parameters that control hydrological fluxes of the model. The legacy storage of NO₃⁻ resulting from agricultural history in the studied catchment supports the assumption that the main DOC and NO₃⁻ sources behave as infinite pools at the scale of several years. Nevertheless, reproducing the long-term trends in solute concentration would require additional information about DOC and NO₃⁻ trends within the reservoirs.

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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.@en