FACTORS AFFECTING EXPORT OF NITRATE-NITROGEN FROM DRAINAGE BASINS IN SOUTHERN ONTARIO

The export of nitrate-N from 21 watersheds near Toronto, Ontario was measured over a 25-month period. The annual average loss ranges from 1.41 to 7.31 kg ha-1. Analysis of these data indicated a very significant correlation between nitrate exports and percentage of each watershed in crops and abandoned agricultural land. A number of soil and topographic variables also had significant correlations with stream nitrate levels. The causal relationships underlying these simple correlations are difficult to evaluate because of considerable multicollinearity between land use and watershed physiography. The separate analysis of stream discharge indicated that agricultural watersheds have lower mean annual discharges (1 s-1 km-2) than those watersheds containing extensive areas of forest and abandoned agricultural land. Variations in annual discharge therefore tended to reduce the contrast in nitrate exports between agricultural and non-agricultural watersheds. The influence of watershed characteristics on nitrate levels exhibited considerable seasonal contrasts. During summer low flow conditions nitrate concentration and loss were not correlated with watershed land use, and instead were affected primarily by localised pollution sources and by biological processes within the streams. © 1978.