A study of stream base flow and NO₃-N concentration was conducted simultaneously in 51 subwatersheds within the 116-square-kilometer watershed of East Mahantango Creek near Klingerstown, Pennsylvania. The study was designed to test whether measurable results of processes and observations within the smaller watersheds were similar to or transferable to a larger scale. Ancillary data on land use were available for the small and large watersheds. Although the source of land-use data was different for the small and large watersheds, comparisons showed that the differences in the two land-use data sources were minimal. A land use-based water-quality model developed for the small-scale 7.3-square-kilometer watershed for a previous study accurately predicted NO₃-N concentrations from sampling in the same watershed. The water-quality model was modified and, using the imagery-based land use, was found to accurately predict NO₃-N concentrations in the subwatersheds of the large-scale 116-square-kilometer watershed as well. Because the model accurately predicts NO₃-N concentrations at small and large scales, it is likely that in second-order streams and higher, discharge of water and NO₃-N is dominated by flow from smaller first-order streams, and the contribution of ground-water discharge to higher order streams is minimal at the large scale.