Methods to predict the agricultural contribution to catchment nitrate loads: designation of nitrate vulnerable zones in Northern Ireland

The European Court of Justice has ruled that the Nitrates Directive places an obligation on EU member states to designate the catchments of eutrophic waters as Nitrate Vulnerable Zones (NVZs), where agriculture is a significant source of nitrate pollution. Recently, the European Commission has indicated that NVZs should be designated when agriculture can be shown to contribute more than 20% of the overall nitrate loading to those waters. Two methods have been used to identify catchments in Northern Ireland, where this condition is met. The first method employs a GIS to predict diffuse losses of nitrate from agricultural land to surface waters based on livestock data and a leaching factor to apportion the annual fertiliser usage and manure between all major catchments. The second method is based on export coefficient modelling. In the first method, allowances for denitrification losses, together with inputs of N from the atmosphere and domestic sewage to surface waters, were made to apportion the total nitrate load to each major river and thus determine if the catchment satisfied the 20% criterion to be designated as a NVZ. Both methods were validated against existing river monitoring data and offered a level of prediction that provides adequate information for a preliminary screening of those catchments to be targeted for designation. Apart from the River Lagan catchment, which includes the city of Belfast and where agriculture only contributed about 8.5% to the nitrate load, the agricultural contribution in the remaining catchments ranged from 33 to 86%. This implies that, if eutrophication is present in the latter surface waters, most of Northern Ireland should be designated a NVZ.