Factors Affecting Stream Nutrient Loads: A Synthesis of Regional SPARROW Model Results for the Continental United States1

Stephen D Preston,Richard B Alexander,Charles G Crawford,Gregory E Schwarz

doi:10.1111/j.1752-1688.2011.00577.x

Stephen D Preston, Richard B Alexander + Show 2 more

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https://doi.org/10.1111/j.1752-1688.2011.00577.x

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Abstract

We compared the results of 12 recently calibrated regional SPARROW (SPAtially Referenced Regressions On Watershed attributes) models covering most of the continental United States to evaluate the consistency and regional differences in factors affecting stream nutrient loads. The models – 6 for total nitrogen and 6 for total phosphorus – all provide similar levels of prediction accuracy, but those for major river basins in the eastern half of the country were somewhat more accurate. The models simulate long-term mean annual stream nutrient loads as a function of a wide range of known sources and climatic (precipitation, temperature), landscape (e.g., soils, geology), and aquatic factors affecting nutrient fate and transport. The results confirm the dominant effects of urban and agricultural sources on stream nutrient loads nationally and regionally, but reveal considerable spatial variability in the specific types of sources that control water quality. These include regional differences in the relative importance of different types of urban (municipal and industrial point vs. diffuse urban runoff) and agriculture (crop cultivation vs. animal waste) sources, as well as the effects of atmospheric deposition, mining, and background (e.g., soil phosphorus) sources on stream nutrients. Overall, we found that the SPARROW model results provide a consistent set of information for identifying the major sources and environmental factors affecting nutrient fate and transport in United States watersheds at regional and subregional scales.

Highlights

Excessive nutrient loading is well established as the primary cause of eutrophication of coastal estuaries as well as freshwater streams and lakes (NRC, 2000)
The United States Geological Survey’s (USGS) SPAtially Referenced Regressions On Watershed attributes (SPARROW) model was developed to aid in the interpretation of monitoring data and simulate water-quality conditions in streams across large spatial scales (Smith et al, 1997)
We present a synthesis of the calibration and simulation results from 12 independently calibrated regional-scale SPARROW models that describe waterquality conditions throughout major drainage basins of the conterminous U.S We find that the results of our analysis of the regional model predictions are generally consistent with our expectations that the predominant sources and processes controlling nutrient loads in streams display many similarities across broad regions of the U.S the analysis reveals new insights about important geographical differences in these controls and their effects on the response of stream nutrient loads as described by regional differences in the model coefficients and predictions of nutrient yields and sources

Summary

Introduction

Excessive nutrient loading is well established as the primary cause of eutrophication of coastal estuaries as well as freshwater streams and lakes (NRC, 2000). Mathematical models provide a means of interpreting and extrapolating monitoring data to mitigate limitations in the quantity of available data and to improve understanding of the environmental factors that affect water quality over large spatial scales and diverse geographic settings. The United States Geological Survey’s (USGS) SPAtially Referenced Regressions On Watershed attributes (SPARROW) model was developed to aid in the interpretation of monitoring data and simulate water-quality conditions in streams across large spatial scales (Smith et al, 1997). SPARROW has been used previously to assess stream nutrient loading for the continental United States (U.S.) (Smith et al, 1997; Smith and Alexander, 2000), regionally for large watersheds in the U.S (e.g., Preston and Brakebill, 1999; Alexander et al, 2000, 2008), and internationally (Alexander et al, 2002). New regional-scale SPARROW nutrient models for the conterminous U.S have been recently developed and are highlighted in the Featured Collection of papers in this issue of the Journal of the American Water Resources Association (Preston et al, 2009; Preston et al, this issue)

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