Richard B. Alexander(1), Richard A. Smith(1), & Gregory E. Schwarz(1), Stephen D. Preston(2),
John W. Brakebill(2), Raghavan Srinivasan(3), and Percy A. Pacheco(4)
1 U.S. Geological Survey, 413 National Center, Reston, Virginia 20192, USA
2 U.S. Geological Survey, Water Resources Division, 8987 Yellow Brick Road, Baltimore, Maryland 21237
3 Blackland Research Center, Texas Agricultural Experiment Station, Texas A&M University, 808 East Blackland Road,Temple, Texas 76502
4 National Oceanographic and Atmospheric Administration, National Ocean Service, Special Projects Office, 1305 East/West Highway, SSMC4, Silver Spring, Maryland 20910
ABSTRACT
To assess the atmospheric contributions of total nitrogen (TN) in riverine exports to coastal and estuarine ecosystems in the United States, we applied a nationally calibrated empirical watershed model, SPARROW (Spatially Referenced Regression on Watershed attributes), to a selected set of 40 major coastal watersheds. In contrast to conventional statistical watershed models, SPARROW uses a mechanistic structure in the correlation of observations of stream nitrogen load with spatial data on contaminant sources, landscape characteristics, and stream properties, allowing separate estimation of the quantities of nitrogen delivered to streams and the outlets of watersheds from point and diffuse sources. We calibrated the model using data from a national set of 374 watersheds. Application of the model to the 40 coastal watersheds indicates that atmospheric nitrogen contributions to riverine export range over nearly two orders of magnitude, from 4 to 326 kg km-2 yr-1. The atmosphere is estimated to contribute from 4 to 35 percent of the TN in stream export with a median of 20 percent. The highest atmospheric contributions are observed in the northeastern and Mid-Atlantic watersheds of the United States. Uncertainties in the estimates, based on the standard error of prediction, range from 40 to 100 percent and vary inversely with watershed size. Agricultural sources typically contribute the largest share of nitrogen (more than one third in most basins), followed by the aggregate contributions of other diffuse sources. Municipal and industrial point sources are similar in magnitude to atmospheric contributions in most watersheds, but represent the largest share (35-88%) of nitrogen in one half of the North Atlantic watersheds and in several watersheds of the Gulf region. Comparisons of the SPARROW model with other national and regional watershed models indicate general agreement in the predictions of TN export over a wide range of watershed sizes. Assessments of atmospheric sources to coastal watersheds are likely to benefit from a continued effort to integrate the mechanistic descriptions of deterministic models with the empirical methods of estimating watershed-scale rate processes and their uncertainties in statistical models.
Additional Information
Additional information on the USGS SPARROW model, from which estimated nitrogen movements in
streams and rivers are derived, is available from the entry page
of the SPARROW Web site.