Modelling the effect of the spatial distribution of agricultural practices on nitrogen fluxes in rural catchments

An integrated, hydrology and nitrogen dynamics model was developed to study the spatial interactions between soil and groundwater that can affect the nitrogen delivery to streamwater in rural catchments. The hydrological model TNT is based on TOPMODEL hypotheses but is it fully distributed according to a regular square grid. A subsurface flow component was distinguished to account for the supply of groundwater and nitrate to downslope soils. The crop growth and nitrogen biotransformations were simulated using an existing generic crop model, STICS. Both models are process-based, but kept as simple as possible. The integrated model was applied to theoretical catchments to analyze the combined effects of geomorphology and crop distribution on the whole catchment nitrogen budget. The catchments differed both in the slope profile and in the pattern of water pathways. The results suggest that placing crops acting as nitrogen sinks downslope potentially polluting crops could reduce significantly the streamwater contamination by nitrate. This effect is the highest for catchments with parallel water pathways and a wide concave bottomland. Nitrogen uptake by sink crops was quantitatively more important than denitrification to reduce nitrogen output. It is concluded that this model, although still in development, may prove an interesting working tool to investigate the effect of the landscape structure on nutrient budgets in ecosystems.