Is a simple model based on two mixing reservoirs able to reproduce the intra-annual dynamics of DOC and NO3 stream concentrations in an agricultural headwater catchment?

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.

Modeling the potential benefits of catch-crop introduction in fodder crop rotations in a Western Europe landscape

Among possible mitigation options to reduce agricultural-borne nitrate fluxes to water bodies, introduction of catch crop before spring crops is acknowledged as a cost-efficient solution at the plot scale, but it was rarely assessed at the catchment level. This study aims to evaluate a set of catch crop implantation scenarios and their consequences in a coastal catchment prone to eutrophication. The objectives are (i) to discuss the potential benefits of catch crop introduction taking into account the limitations due to the physiographic and agricultural context of the area (ii) to propose a multicriteria classification of these scenarios as a basis for discussion with stakeholders. We used the distributed agro-hydrological model TNT2 to simulate 25 scenarios of catch crop management, differing in length of catch crop growing period, place in the crop rotation and residue management. The scenarios were classified considering the variations in main crop yields and either nitrogen fluxes in stream or the global nitrogen mass balance at the catchment level. The simulations showed that in the catchment studied, little improvement can be expected from increasing the catch crop surface. Catch crop cultivation was always beneficial to reduce nitrogen losses, but led to adverse effects on main crop yields in some cases. Among the scenarios involving additional catch crop surface, introducing catch crop between two winter cereals appeared as the most promising. The classification of scenarios depended on the chosen criteria: when considering only the reduction of nitrogen fluxes in streams, exporting catch crop residues was the most efficient while when considering the global nitrogen mass balance, soil incorporation of catch crop residues was the most beneficial. This work highlights the interest, while using integrated models, of assessing simulated scenarios with multicriteria approach to provide stakeholder with a picture as complete as possible of the consequences of prospective policies.

NitroScape: a model to integrate nitrogen transfers and transformations in rural landscapes

Modelling nitrogen transfer and transformation at the landscape scale is relevant to estimate the mobility of the reactive forms of nitrogen (N(r)) and the associated threats to the environment. Here we describe the development of a spatially and temporally explicit model to integrate N(r) transfer and transformation at the landscape scale. The model couples four existing models, to simulate atmospheric, farm, agro-ecosystem and hydrological N(r) fluxes and transformations within a landscape. Simulations were carried out on a theoretical landscape consisting of pig-crop farms interspersed with unmanaged ecosystems. Simulation results illustrated the effect of spatial interactions between landscape elements on N(r) fluxes and losses to the environment. More than 10% of the total N(2)O emissions were due to indirect emissions. The nitrogen budgets and transformations of the unmanaged ecosystems varied considerably, depending on their location within the landscape. The model represents a new tool for assessing the effect of changes in landscape structure on N(r) fluxes.