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Variability of Annual Nutrient and Sediment Discharges in Runoff from Oklahoma Cropland and Rangeland¹

ABSTRACT

Nitrogen and phosphorus discharges in runoff from nearly level cropland and 3% sloping rangeland were measured from July 1972, to June 1976. Sediment discharges and runoff amounts from these 5- to 18-ha watersheds were measured from July 1966, to June 1976. Sediment and nutrient discharges varied greatly from year to year and between different land uses. We concluded that long records are needed to compare discharges from different management practices. The average and maximum annual sediment discharges, respectively, were 3,600 and 8,900 kg/ha from irrigated cotton (Gossypium hirsutum), 900 and 3,900 kg/ha from dryland wheat (Triticum aestivum), 400 and 1,800 kg/ha from range with limited grazing, and 9,000 and 23,000 kg/ha from overgrazed range. Maximum annual sediment discharges occurred during the period in which nutrient discharges were measured. Maximum annual nutrient discharges were 13 kg/ha total N, 4 kg/ha nitrate N, 11 kg/ha total P, and 2 kg/ha soluble P. The average annual discharge for each nutrient form and land use was about half of its maximum value. Nitrate accounted for 10 to 30% of the total N discharged. Soluble phosphate accounted for about 20% of the total P discharged from cropland, and <10% of that discharged from rangeland. Annual deposition in rainfall averaged 5 kg/ha N and 0.15 kg/ha P.

Key Words: water quality • fertilizer use • nitrogen • phosphorus • soluble nutrients • rainfall composition • sediment yield

¹ Contribution from the USDA-SEA Agric. Water Qual. Manage. Lab., Durant, OK 74701, in cooperation with the USDA-SEA Southern Great Plains Watershed Res. Center, Chickasha, OK 73018 and the Oklahoma Agric. Exp. Stn., Stillwater, OK 74074.

² Director, Agric. Engineer, and Soil Scientists, respectively, USDA-SEA Agric. Water Quality Manage. Lab.

Received for publication December 22, 1977.

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