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Volume 37, Issue 5 p. 1862-1874
Surface Water Quality

Water Quality Trends and Changing Agricultural Practices in a Midwest U.S. Watershed, 1994–2006

William H. Renwick

Corresponding Author

William H. Renwick

Geography Dep., Miami Univ., Oxford, OH, 45056

Corresponding author ([email protected]).Search for more papers by this author
Michael J. Vanni

Michael J. Vanni

Zoology Dep., Miami University, Oxford, OH, 45056

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Qianyi Zhang

Qianyi Zhang

Dep. of Statistics, North Carolina State Univ., Raleigh, NC, 27695

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Jon Patton

Jon Patton

Center for the Advancement of Computational Research, Miami Univ., Oxford, OH, 45056

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First published: 01 September 2008
Citations: 40

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Abstract

Sediment and nutrient concentrations in surface water in agricultural regions are strongly influenced by agricultural activities. In the Corn Belt, recent changes in farm management practices are likely to affect water quality, yet there are few data on these linkages at the landscape scale. We report on trends in concentrations of N as ammonium (NH4) and nitrate (NO3), soluble reactive phosphorus (SRP), and suspended sediment (SS) in three Corn Belt streams with drainage areas of 12 to 129 km2 for 1994 through 2006. During this period, there has been an increase in conservation tillage, a decline in fertilizer use, and consolidation of animal feeding operations in our study watersheds and throughout the Corn Belt. We use an autoregressive moving average model to include the effects of discharge and season on concentrations, LOWESS plots, and analyses of changes in the relation between discharge and concentration. We found significant declines in mean monthly concentrations of NH4 at all three streams over the 13-yr period, declines in SRP and SS in two of the three streams, and a decline in NO3 in one stream. When trend coefficients are converted to percent per year and weighted by drainage, area changes in concentration are −8.5% for NH4, −5.9% for SRP, −6.8% for SS, and −0.8% for NO3 Trends in total N and P are strongly tied to trends in NO3, SRP, and SS and indicate that total P is declining, whereas total N is not.