PREDICTING NITRATE CONCENTRATIONS IN NORTHERN-IRELAND RIVERS USING TIME-SERIES ANALYSIS

Lough Neagh is an important natural resource, being the largest single source of potable water for Belfast. Its catchment area covers about one-third of the land area of Northern Ireland. Six inflow rivers drai n 88% of the total catchment, which has a land usage of grassland and rough grazing that is similar to Northern Ireland as a whole. This rep ort examines the causes of the year-to-year variation in river NO3- co ncentrations that have been the focus of European Community (EC) legis lation. A multiple regression model has been developed which explains 74% of the variation in NO3- concentrations for the time period 1974 t o 1994. Based on a knowledge of the Northern Ireland annual utilized m etabolizable energy (UME) for grazing livestock and N fertilizer sales , the model employs the efficiency of fertilizer usage as an independe nt variable. Additional independent variables are climatic: previous s ummer rainfall, sun hours in the winter and rainfall for the period Ja nuary to June. Based on a worst case scenario, employing the extreme v alues for these climatic variables and the least fertilizer efficiency ratio, the predicted NO3- concentration is only 4.4 mg N L-1, which i s well below the EC maximum admissible level of 11.3 mg N L-1 for drin king water.