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Transport and Prediction of Sulfate in Agricultural Runoff

USDA-ARS, Water Quality and Watershed Res. Lab., P. O. Box 1430, Durant, OK 74702-1430;

O. R. Jones

USDA-ARS, Conserv. and Production Res. Lab., Drawer 10, Bushland, TX 79012;

W. A. Berg

USDA-ARS, Southern Plains Range Exp. Stn., 2000 18th Street, Woodward, OK 73801;

G. A. Coleman

Southern Plains Watershed and Water Quality Lab., P. O. Box 400, Chickasha, OK 73018.

^* Corresponding author.

ABSTRACT

The measurement and simulation of sulfate-S (SO₄-S) mobility in agricultural watersheds is necessary to evaluate the effect of management practices on associated crop S deficiencies, enhanced leaching loss of nutrient cations, and acidification of percolation waters. The concentrations and amounts of SO₄-S in runoff from six grassed and 13 cropped watersheds in the Southern Plains were, thus, measared over a 4-yr period. Sulfate-S transport in runoff was predicted using an equation describing the kinetics of SO₄-S desorption from soil and compared with measured values. No SO₄-S was added to any of the watersheds directly as S fertilizer or indirectly in N or P fertilizer material. No difference (at 5% level) in SO₄-S concentration in runoff from grassed (mean annual value of 12.6 mg L^–1) and cropped (mean annual value of 11.0 mg L^–1) watersheds was observed. Differences in amounts (0.2–18.9 kg ha^–1 yr^–1) were a function of runoff volume as influenced by land management. A general trend of increasing SO₄-S concentration in runoff with decreasing pH was observed, which may be a function of S dry deposition and soil and crop conditions. Measured and predicted SO₄-S concentrations in runoff for individual events were not significantly different (at 5% level), with an average predictive standard error of 1.6 mg L^–1 for all watersheds, representing 17% of the measured concentration. The equation may, thus, provide a predictive tool in agronomic and environmental studies of SO₄-S movement in agricultural watersheds.