Modelling of Nitrate Leaching on a Regional Scale Using a GIS

A GIS was used to model nitrate leaching losses to surface waters in Northern Ireland by integrating a number of database, including farm and population census data, on a 10 × 10 km grid. This model enabled an estimate to be made of the contribution of agriculture, domestic sewage and rainfall deposition to the amount of nitrate leached from each grid square. The model was also used to predict annual mean nitrate concentration within each grid square and the predictions validated against data from a statutory river monitoring programme. Predicted annual flow rates and nitrate loadings to watercourses from catchments ranging in size from 14·9 km2 to 4453 km2 were compared with estimates based on conventional monitoring. In general, the predicted flows and loads agreed with observed values to within ±10% for flow rates and to within ±25% for nitrate loads. This level of prediction is an adequate level on which to base management decisions covering possible designation under EC directives. The model predicted that losses of nitrate from agriculture accounted for around 70% of the annual total nitrate load to a major watercourse. In the easter counties of Northern Ireland, rainfall and domestic sewage made approximately equal contributions to the remaining 30% of the nitrate leached. However, in the less heavily populated western counties of Northern Ireland, the contribution from rainfall to the amount of nitrate leached was around five times that from domestic sewage.