Targeted management of a small number of catchments may help reduce nitrogen loading to Chesapeake Bay

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Largest projected reductions associated with decreasing agricultural fertilizer application

Watershed models can help develop management solutions to thorny water-quality problems, such as how to decrease nitrogen loads to critical water bodies. 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—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. Nitrogen loading contributes to the formation of a seasonal hypoxic low-oxygen or “dead” zone that can kill aquatic life. The SPARROW model provided insight into how nitrogen loading 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.

Image: Susquehanna River - Conowingo Dam

Large rivers, like the Susquehanna River, shown here, deliver nitrogen to Chesapeake Bay. Nitrogen loading to the Bay could be reduced by an estimated 25%, meeting the requirements of the USEPA Total Maximum Daily Load (TMDL), by eliminating fertilizer applied to the 7% of sub-watersheds that contribute the greatest nitrogen loads with a fertilizer source. (Photo credit: Wendy McPherson, USGS)

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