DEVELOPMENT AND APPLICATION OF A NITROGEN MODELLING SYSTEM FOR LARGE CATCHMENTS

The NELUP Nitrogen Modelling System (NMS) has been developed to model the transport of nitrates within large catchments. It is one component of a suite of hydrological models that have been integrated with ecological and agroeconomic models in a Decision Support System. The NMS has been designed to be applied in four stages: determination of nitrogen input categories; simulation of plant uptake and decay using the farm management model EPIC; simulation of initial nitrate profiles in the unsaturated zone with the one-dimensional transport model (MP); and finally a simulation of nitrate transport using the SHETRAN-UK catchment flow and transport modelling system. The NMS has been applied to model the transport of nitrate within the 3000 km2 Tyne basin for the period 1985–1989. Predicted in-stream nitrate concentrations throughout the catchment display a low base concentration with peaks corresponding to overland flow events following fertiliser application. Model results compare favourably with the available monthly NRA nitrate observation data at a number of different locations within the catchment. A short field study has been undertaken within the Tyne which demonstrates the occurrence of the transport of nitrate within overland flow events, a hitherto undocumented process.