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A simple index explains annual atrazine transport from surface runoff-prone watersheds in the north-central USA

Sadler, E. J., Sudduth, K. A., Lerch, R. N., Baffaut, C., Kitchen, N. R.
Hydrological processes 2014 v.28 no.2 pp. 210
agricultural runoff, agricultural watersheds, application rate, application timing, atrazine, growing season, herbicide residues, nonpoint source pollution, pesticide application, streams, weather, Missouri
Year-to-year dynamics in weather affect both the timing of application and the potential hydrologic transport of pesticides. Further, the most commonly used pesticides dissipate in the environment during the growing season. Interactions among these factors – hydrology, timing of application and dissipation kinetics – hinder the detection of temporal trends in transport. It is increasingly important to be able to discern such trends, to judge effectiveness of management practices or to determine whether observed changes were caused by management or weather. In previous work, a cumulative vulnerability index was developed to account for these three factors. It explained 63% of annual variation in atrazine load in the Goodwater Creek Experimental Watershed (GCEW). The objectives of the current work were (i) to generalize the cumulative vulnerability index to explicitly account for variation in watershed size, area treated with atrazine and average application rate; (ii) to test the overall performance on watersheds showing such variation; and (3) to test whether the generalized index properly accounted for the additional input parameters. The generalized index was tested using data from GCEW (73.7 km2) and seven additional watersheds in the northeast Missouri claypan region that varied in size from 212 to 1180 km2 and from 4% to 23% of watershed area planted to corn or sorghum. Across 32 site-years, the generalized index explained 84% of variation in annual atrazine load. Further, tests of residuals showed no dependence on either watershed area or fraction of area planted to corn and sorghum, indicating that these parameters were properly integrated into the index. The performance of the index supports the conclusion that data obtained from GCEW is representative of the Mark Twain Lake Basin and likely the entire Central Claypan Major Land Resource Area.