Tilted‐bed Simulation of Erosion and Chemical Runoff from Agricultural Fields: I. Runoff of Sediment and Sediment‐associated Copper and Zinc

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Abstract
Pesticide loss from agricultural fields by runoff was simulated using a rainfall simulator over 2.2‐ by 1.2‐m fiberglass “tilted bed” soil trays. Intense rainfall events were applied to eight trays containing a loam soil at 1.3 ± 0.4% slope. To simulate soil‐surface‐applied pesticides thai are lost in the sediment phase of runoff, the soil surfaces of six of the trays (two were used as controls) were sprayed with aqueous solutions of Cu and Zn salts to give “application rates” of 1 kg elemental Cu and Zn/ha. The rainfall simulator applied 30 mm of rain to the trays in 5 min with a CV of 10%, and an average of 7.6 mm of runoff was obtained with a CV of 14%. The trays that received salt applications showed significantly lower erosion rates (629 ± 160 kg/ha) than those that did not receive salts (1200 ± 100 kg/ha), even though the total salt applied was only 11 kg/ha. All measurable Cu and Zn was found in the sediment phase of the runoff, and enrichment of the metals in the sediment was a function of runoff flow rate. The estimated adsorption constants (Kd) for Cu and Zn in this soil were 186 and 20, respectively, but this difference did not effect runoff losses; the total loads of Cu and Zn from individual trays, after correction for background Cu and Zn, were essentially identical and highly correlated with soil loss. The total losses of the tracers ranged from 3 to 7% of the amounts applied, in good agreement with field studies of losses of pesticides strongly bound to soil. These results suggest that this system may provide an inexpensive alternative to field testing for pesticide mobility in runoff, and provide insights into the physical process by which pesticides are entrained into runoff.
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