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Spatial Analysis of Instream Nitrogen Loads to Streams in the Southeastern United States

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Spatial Analysis of Instream Nitrogen Loads and Factors Controlling Nitrogen Delivery to Streams in the Southeastern United States Using Spatially Referenced Regression on Watershed Attributes (SPARROW) and Regional Classification Frameworks

By Anne B. Hoos and Gerard McMahon

The official version of this journal article is available from John Wiley & Sons, Ltd., and was published as:
Hoos, A.B., and McMahon, Gerard, 2009, Spatial analysis of instream nitrogen loads and factors controlling nitrogen delivery to streams in the southeastern United States using spatially referenced regression on watershed attributes (SPARROW) and regional classification frameworks: Hydrologic Processes, doi:10.1002/hyp.7323, accessed June 25, 2009, at http://dx.doi.org/10.1002/hyp.7323


PDF of article (publicly available): Spatial analysis of instream nitrogen loads and factors controlling nitrogen delivery to streams in the southeastern United States using spatially referenced regression on watershed attributes (SPARROW) and regional classification frameworks (0.77 MB)

Abstract

Understanding how nitrogen transport across the landscape varies with landscape characteristics is important for developing sound nitrogen management policies. We used a spatially referenced regression analysis (SPARROW) to examine landscape characteristics influencing delivery of nitrogen from sources in a watershed to stream channels. Modelled landscape delivery ratio varies widely (by a factor of 4) among watersheds in the southeastern United States—higher in the western part (Tennessee, Alabama, and Mississippi) than in the eastern part, and the average value for the region is lower compared to other parts of the nation. When we model landscape delivery ratio as a continuous function of local-scale landscape characteristics, we estimate a spatial pattern that varies as a function of soil and climate characteristics but exhibits spatial structure in residuals (observed load minus predicted load). The spatial pattern of modelled landscape delivery ratio and the spatial pattern of residuals coincide spatially with Level III ecoregions and also with hydrologic landscape regions. Subsequent incorporation into the model of these frameworks as regional scale variables improves estimation of landscape delivery ratio, evidenced by reduced spatial bias in residuals, and suggests that cross-scale processes affect nitrogen attenuation on the landscape. The model-fitted coefficient values are logically consistent with the hypothesis that broad-scale classifications of hydrologic response help to explain differential rates of nitrogen attenuation, controlling for local-scale landscape characteristics. Negative model coefficients for hydrologic landscape regions where the primary flow path is shallow ground water suggest that a lower fraction of nitrogen mass will be delivered to streams; this relation is reversed for regions where the primary flow path is overland flow.

Supporting Information

Supporting information_20090607.doc (0.035 MB)
Includes brief descriptions of the two data sets of model output, and instructions for acquiring the geospatial data that can be used to map the model output.

predictions_modelB_20090504.txt (2.39 MB)
Tab-delimited data set containing model predictions (from Model B) of instream load, source shares of instream load, load delivered to the stream from the adjacent catchment, delivery variation factor, and fraction delivered to the basin outlet.

predictions_modelB_20090504.xls (4.13 MB)
Excel version of predictions_modelB_20090504.txt

predictions_modelB_README.txt (0.012 MB)
Tab-delimited file containing descriptions of each of the attributes reported in predictions_modelB_20090504.txt

predictions_modelB_README.xls (0.036 MB)
Excel version of predictions_modelB_README.txt

summarysourceshares_modelB.txt (0.003 MB)
Tab-delimited summary table of source shares of load delivered to streams and to the basin outlet

summarysourceshares_modelB.xls (0.026 MB)
Excel version of summarysourceshares_modelB.txt

instreamload_modelsABC_20090608.txt (0.79 MB)
Tab-delimited data set containing predictions of instream load from three separate model specifications, illustrating the effect of the regional landscape variables on estimated delivery of nitrogen to streams

instreamload_modelsABC_20090608.xls (1.75 MB)
Excel version of instreamload_modelsABC_20090608.txt

instreamload_modelsABC_README.txt (0.004 MB)
Tab-delimited file containing descriptions of each of the attributes reported in instreamload_modelsABC_20090608.txt

instreamload_modelsABC_README.xls (0.019 MB)
Excel version of instreamload_modelsABC_README.txt

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

For additional information contact:
Anne B. Hoos, USGS Tennessee Water Science Center
640 Grassmere Park, Suite 100
Nashville, TN 37211
(615) 837–4760

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