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TECHNICAL REPORTS

Surface Water Quality

Modeling Riverine Nitrate Export from an East-Central Illinois Watershed Using SWAT

^a Dep. of Civil and Environmental Engineering, 4158 Newmark Lab., 205 North Mathews Avenue, Urbana, IL
^b Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois at Urbana Champaign W-503 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL

^* Corresponding author (xuetaohu{at}uiuc.edu)

Received for publication June 14, 2006. Reliable water quality models are needed to forecast the water quality consequences of different agricultural nutrient management scenarios. In this study, the Soil and Water Assessment Tool (SWAT), version 2000, was applied to simulate streamflow, riverine nitrate (NO₃) export, crop yield, and watershed nitrogen (N) budgets in the upper Embarras River (UER) watershed in east-central Illinois, which has extensive maize-soybean cultivation, large N fertilizer input, and extensive tile drainage. During the calibration (1994–2002) and validation (1985–1993) periods, SWAT simulated monthly and annual stream flows with Nash-Sutcliffe coefficients (E) ranging from 0.67 to 0.94 and R² from 0.75 to 0.95. For monthly and annual NO₃ loads, E ranged from –0.16 to 0.45 and R² from 0.36 to 0.74. Annual maize and soybean yields were simulated with relative errors ranging from –10 to 6%. The model was then used to predict the changes in NO₃ output with N fertilizer application rates 10 to 50% lower than original application rates in UER. The calibrated SWAT predicted a 10 to 43% decrease in NO₃ export from UER and a 6 to 38% reduction in maize yield in response to the reduction in N fertilizer. The SWAT model markedly overestimated NO₃ export during major wet periods. Moreover, SWAT estimated soybean N fixation rates considerably greater than literature values, and some simulated changes in the N cycle in response to fertilizer reduction seemed to be unrealistic. Improving these aspects of SWAT could lead to more reliable predictions in the water quality outcomes of nutrient management practices in tile-drained watersheds.

Abbreviations: CN, curve number • , mean relative error • E, Nash-Sutcliffe coefficient • HRU, hydrologic response unit • MSE, mean squared error • SWAT, Soil and Water Assessment Tool • UER, Upper Embarras River