Nitrogen from Agriculture and Urban Activities
Nitrogen is present in crop fertilizer and livestock manure, as well as residential lawn fertilizers and other urban sources. This nitrogen can infiltrate to groundwater, or get carried to the nearest waterbody via stormwater runoff or overland flow.
Small streams draining agricultural and residential lands had much higher nitrogen yields than streams draining forested land (Herrera, 2011, as summarized in Roberts and Kolosseus, 2011), especially during storm events. However, baseflow nitrogen concentrations were also high in residential watersheds. Ebbert et al. (2000) found higher levels of nitrogen from agricultural watersheds in another study of small streams in the Puget Sound basin.
The USGS has developed a regional terrestrial SPARROW model of nitrogen for the Pacific Northwest. Though the model is not optimized for the Puget Sound region, it includes the Puget Sound region, tracks various land-based sources of nitrogen within smaller watersheds and subbasins, and routes these sources to downstream waterbodies.
SPARROW model results presented in Wise and Johnson (2011a) show that the share of total nitrogen load from specific upstream land uses/activities within the Puget Sound basins are as follows:
Developed land: 16.8%
Livestock manure: 10.9%
Farm fertilizer: 2.9%
As described in Nitrogen in Stormwater Runoff, we also have estimates of aerial nitrogen loading rates during storm events from agricultural, residential, and commercial/industrial land areas within watersheds i.e. the mass of nitrogen from a particular land use activity per unit of land area per year.
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