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Models have become an important tool to select nonpoint source pollution management strategies. The Agricultural Non-Point Source Pollution model (AGNPS) was used to evaluate the effect of cell size on the estimation of pollutant loads in the Canagagigue Creek watershed of the Grand River, Ontario. The GIS interface of the model (RAISON) was used to extract input parameters from digital elevation model, soils and land use layers. The cell size used was 25, 50, 100, 150, 300, and 500 ha with storm return period of 2,5,10, and 25 years.

An interesting finding of the study was that the size of cells in the GIS model has a significant effect on the model predictions. The runoff and sediment yield predicted by the model observed at the sub-watershed outlet and the watershed outlet showed erratic pattern with an increase in cell size. There was increase in estimated runoff volume and sediment yields when cell size increased from 25 ha to 100 ha, again increase in cell size from 100 to 150 ha showed decrease in runoff volume and sediment yield. Further increase in cell size greater than 150 ha showed an increase in runoff volume and sediment yield. Particular care must be dedicated to selection of cell size and its effect on the AGNPS model results.