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TECHNICAL REPORTS

Surface Water Quality

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

^a Geography Dep., Miami Univ., Oxford, OH, 45056
^b Zoology Dep., Miami University, Oxford, OH, 45056
^c Dep. of Statistics, North Carolina State Univ., Raleigh, NC 27695
^d Center for the Advancement of Computational Research, Miami Univ., Oxford, OH, 45056

^* Corresponding author (renwicwh{at}muohio.edu).

Received for publication July 27, 2007. 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.

Abbreviations: ARIMA, autoregressive moving average • SRP, soluble reactive phosphorus • SS, suspended sediment • UFMC, Upper Four Mile Creek