Influence of climate and human activities on the relationship between watershed nitrogen input and river export.

River export of nitrogen (N) is influenced strongly by spatial variation in anthropogenic N inputs and climatic variation. We developed a model of riverine N export for 18 Lake Michigan Basin watersheds based on N budgets at 5-year intervals from 1974 to 1992. N inputs explained a high proportion of the spatial variation in river export but virtually none of the temporal variation, whereas between year N export was related to variation in discharge for over one-half of the rivers. A regression model of riverine N exports as an exponential function of N inputs and a powerfunction of annual water discharge accounted for 87% of the variation in annual total nitrogen fluxes over space and time. Application of this model to three scenarios of future land use, including business as usual, greater reliance on organic farming methods, and expanded corn-based ethanol production, and two climate scenarios, including increases in water discharge by 5% and 10%, suggests that riverine N export is likely to increase by as much as 24% in response to heavierfertilizer use for expanded corn production and a 10% increase in annual discharge. However, N export by rivers could decrease below present-day export through reduced reliance on commercial fertilizer use.