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January 7, 2020

A new USGS study uses the SPARROW (SPAtially Referenced Regression On Watershed attributes) model to assess how nitrogen loading to the Chesapeake Bay might change in response to changing different sources of nitrogen inputs. The largest reduction in load is predicted to occur if nitrogen fertilizer applied to agricultural land is decreased.

The clock is ticking on reductions in nitrogen delivered to Chesapeake Bay—the U.S. Environmental Protection Agency total maximum daily load (TMDL) requires that land-management practices expected to result in a 25% reduction (from 2009 levels) in nitrogen load must be in place by 2025. Watershed models can help develop management solutions to thorny water-quality problems, such as how to decrease nitrogen loads to critical water bodies.

Nitrogen loading contributes to the formation of a seasonal hypoxic low-oxygen or “dead” zone that can kill aquatic life. A USGS SPARROW model provides insight into how nitrogen loading to Chesapeake Bay is likely to respond to changes in source inputs and land use.

USGS scientists used the SPARROW model to identify those catchments draining to Chesapeake Bay where land-management strategies for reducing nitrogen loading would most effectively reduce future watershed-scale nitrogen loading. The model results estimate that TMDL goals could be met by various changes to fertilizer applications. For example, decreasing the mass of nitrogen applied as fertilizer to all agricultural lands in the catchment by approximately 50% could meet TMDL goals. Alternatively, eliminating all fertilizer applied to the 7% of catchments that contribute the greatest fertilizer-sourced nitrogen loads to the Bay could also meet TMDL goals. Reduction of nitrogen fertilizer applications was predicted to result in a much greater decrease in loading than other reduction scenarios, such as reductions in nitrogen point-source inputs, atmospheric deposition, and manure application to agricultural lands.

Citation: Miller, M., Capel, P.D., Garcia, A.M., and Ator, S. Response of nitrogen loading to the Chesapeake Bay to source reduction and land use change scenarios: A SPARROW‐informed analysis. Journal of the American Water Resources Association,

Learn more about SPARROW models and try out interactive mappers here.

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