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Scientific Investigations Report 2013–5007

Prepared in cooperation with the North Carolina Department of Environment and Natural Resources, Division of Water Quality

Relation of Watershed Setting and Stream Nutrient Yields at Selected Sites in Central and Eastern North Carolina, 1997–2008

By Stephen L. Harden, Thomas F. Cuffney, Silvia Terziotti, and Katharine R. Kolb

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Abstract

Data collected between 1997 and 2008 at 48 stream sites were used to characterize relations between watershed settings and stream nutrient yields throughout central and eastern North Carolina. The focus of the investigation was to identify environmental variables in watersheds that influence nutrient export for supporting the development and prioritization of management strategies for restoring nutrient-impaired streams.

Nutrient concentration data and streamflow data compiled for the 1997 to 2008 study period were used to compute stream yields of nitrate, total nitrogen (N), and total phosphorus (P) for each study site. Compiled environmental data (including variables for land cover, hydrologic soil groups, base-flow index, streams, wastewater treatment facilities, and concentrated animal feeding operations) were used to characterize the watershed settings for the study sites. Data for the environmental variables were analyzed in combination with the stream nutrient yields to explore relations based on watershed characteristics and to evaluate whether particular variables were useful indicators of watersheds having relatively higher or lower potential for exporting nutrients.

Data evaluations included an examination of median annual nutrient yields based on a watershed land-use classification scheme developed as part of the study. An initial examination of the data indicated that the highest median annual nutrient yields occurred at both agricultural and urban sites, especially for urban sites having large percentages of point-source flow contributions to the streams. The results of statistical testing identified significant differences in annual nutrient yields when sites were analyzed on the basis of watershed land-use category. When statistical differences in median annual yields were noted, the results for nitrate, total N, and total P were similar in that highly urbanized watersheds (greater than 30 percent developed land use) and (or) watersheds with greater than 10 percent point-source flow contributions to streamflow had higher yields relative to undeveloped watersheds (having less than 10 and 15 percent developed and agricultural land uses, respectively) and watersheds with relatively low agricultural land use (between 15 and 30 percent). The statistical tests further indicated that the median annual yields for total P were statistically higher for watersheds with high agricultural land use (greater than 30 percent) compared to the undeveloped watersheds and watersheds with low agricultural land use. The total P yields also were higher for watersheds with low urban land use (between 10 and 30 percent developed land) compared to the undeveloped watersheds. The study data indicate that grouping and examining stream nutrient yields based on the land-use classifications used in this report can be useful for characterizing relations between watershed settings and nutrient yields in streams located throughout central and eastern North Carolina.

Compiled study data also were analyzed with four regression tree models as a means of determining which watershed environmental variables or combination of variables result in basins that are likely to have high or low nutrient yields. The regression tree analyses indicated that some of the environmental variables examined in this study were useful for predicting yields of nitrate, total N, and total P. When the median annual nutrient yields for all 48 sites were evaluated as a group (Model 1), annual point-source flow yields had the greatest influence on nitrate and total N yields observed in streams, and annual streamflow yields had the greatest influence on yields of total P. The Model 1 results indicated that watersheds with higher annual point-source flow yields had higher annual yields of nitrate and total N, and watersheds with higher annual streamflow yields had higher annual yields of total P.

When sites with high point-source flows (greater than 10 percent of total streamflow) were excluded from the regression tree analyses (Models 2–4), the percentage of forested land in the watersheds was identified as the primary environmental variable influencing stream yields for both total N and total P. Models 2, 3 and 4 did not identify any watershed environmental variables that could adequately explain the observed variability in the nitrate yields among the set of sites examined by each of these models. The results for Models 2, 3, and 4 indicated that watersheds with higher percentages of forested land had lower annual total N and total P yields compared to watersheds with lower percentages of forested land, which had higher median annual total N and total P yields. Additional environmental variables determined to further influence the stream nutrient yields included median annual percentage of point-source flow contributions to the streams, variables of land cover (percentage of forested land, agricultural land, and (or) forested land plus wetlands) in the watershed and (or) in the stream buffer, and drainage area. The regression tree models can serve as a tool for relating differences in select watershed attributes to differences in stream yields of nitrate, total N, and total P, which can provide beneficial information for improving nutrient management in streams throughout North Carolina and for reducing nutrient loads to coastal waters.

First posted May 23, 2013

  • Appendixes
    (All Appendixes are MS Excel Files)

    Appendix1. Water quality, streamflow, and nutrient load data by basin and site

    Appendix2. Watershed land cover, stream buffer land cover, hydrologic soil groups, and precipitation data

    Appendix3. NPDES point-source data and CAFO data

    Appendix4. Dataset used for regression tree analyses

    All Appendixes zipped (70.7 MB)

For additional information contact:
Director, North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, NC 27607
(919) 571–4000
http://nc.water.usgs.gov/

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Suggested citation:

Harden, S.L., Cuffney, T.F., Terziotti, Silvia, and Kolb, K.R., 2013, Relation of watershed setting and stream nutrient yields at selected sites in central and eastern North Carolina, 1997–2008: U.S. Geological Survey Scientific Investigations Report 2013–5007, 47 p., http://pubs.usgs.gov/sir/2013/5007/.



Contents

Acknowledgments

Abstract

Introduction

Methods of Data Compilation and Analysis

Watershed Setting

Relation of Streamflow and Nutrient Loads

Selection of Nutrient Yield Data for Statistical Evaluation

Relation of Watershed Setting and Stream Nutrient Yields

Summary and Conclusions

References Cited


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