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TECHNICAL REPORTS

Surface Water Quality

Variable Rainfall Intensity and Tillage Effects on Runoff, Sediment, and Carbon Losses from a Loamy Sand under Simulated Rainfall

^a USDA-ARS, Southeast Watershed Research Lab., Tifton, GA 31793
^b USDA-ARS, J. Phil Campbell Sr., Natural Resource Conservation Center, Watkinsville, GA 30677
^c Univ. of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793

^* Corresponding author (Clint.Truman{at}ars.usda.gov).

Received for publication January 6, 2006. The low-carbon, intensively cropped Coastal Plain soils of Georgia are susceptible to runoff, soil loss, and drought. Reduced tillage systems offer the best management tool for sustained row crop production. Understanding runoff, sediment, and chemical losses from conventional and reduced tillage systems is expected to improve if the effect of a variable rainfall intensity storm was quantified. Our objective was to quantify and compare effects of a constant (I_c) intensity pattern and a more realistic, observed, variable (I_v) rainfall intensity pattern on runoff (R), sediment (E), and carbon losses (C) from a Tifton loamy sand cropped to conventional-till (CT) and strip-till (ST) cotton (Gossypium hirsutum L.). Four treatments were evaluated: CT-I_c, CT-I_v, ST-I_c, and ST-I_v, each replicated three times. Field plots (n = 12), each 2 by 3 m, were established on each treatment. Each 6-m² field plot received simulated rainfall at a constant (57 mm h^–1) or variable rainfall intensity pattern for 70 min (12-run ave. = 1402 mL; CV = 3%). The I_v pattern represented the most frequent occurring intensity pattern for spring storms in the region. Compared with CT, ST decreased R by 2.5-fold, E by 3.5-fold, and C by 7-fold. Maximum runoff values for I_v events were 1.6-fold higher than those for I_c events and occurred 38 min earlier. Values for E_tot and C_tot for I_v events were 19–36% and 1.5-fold higher than corresponding values for I_c events. Values for E_max and C_max for I_v events were 3-fold and 4-fold higher than corresponding values for I_c events. Carbon enrichment ratios (CER) were 1.0 for ST plots and 1.0 for CT plots (except for first 20 min). Maximum CER for CT-I_c, CT-I_v, ST-I_c, and ST-I_v were 2.0, 2.2, 1.0, and 1.2, respectively. Transport of sediment, carbon, and agrichemicals would be better understood if variable rainfall intensity patterns derived from natural rainfall were used in rainfall simulations to evaluate their fate and transport from CT and ST systems.

Abbreviations: C, carbon loss • CER, carbon enrichment ratio • CT, conventional tillage • CV, coefficient of variation • E, sediment yield • I_c, constant rainfall intensity • INF, infiltration • I_v, variable rainfall intensity • P, paratill • R, runoff • Rs, residue • ST, strip tillage

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