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

Authors: Sadler, Edward; Sudduth, Kenneth - Ken; Lerch, Robert; Baffaut, Claire; Kitchen, Newell

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2012
Publication Date: 1/7/2014
Citation: Sadler, E.J., Sudduth, K.A., Lerch, R.N., Baffaut, C., Kitchen, N.R. 2014. A simple index explains annual atrazine transport from surface runoff-prone watersheds in the north-central USA. Hydrological Processes. 28:210-217. DOI: 10.1002/hyp.9544.

Interpretive Summary: Year-to-year dynamics in weather affect both the timing of application and potential hydrologic transport of pesticides. Further, 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 (CVI) was developed to account for these three factors. It explained 63 percent of annual variation in atrazine load in the Goodwater Creek Experimental Watershed (GCEW). In current work, the CVI was generalized to explicitly account for variation in watershed size, area treated with atrazine, and average application rate. The generalized index was tested using data from GCEW (73.7 km2) and 7 additional watersheds in the northeast Missouri claypan region that varied in size from 212 to 1183 km2 and from 4 percent to 23 percent of watershed area planted to corn or sorghum. Across 32 site-years, the generalized index explained 85 percent of variation in annual atrazine load. Further, tests of residuals showed no dependence on either watershed area or fraction of area planted to corn or 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.

Technical Abstract: Detecting water quality effects of conservation practices at a watershed scale is complicated. Beneficial management practices may be very effective at the edge of a field, but their effect is often very difficult to detect at the watershed scale. Further, effectiveness of the practice itself often depends on weather, especially precipitation. And variations in that weather cause wide variations in observations at the watershed scale. All told, detecting trends from noisy watershed-scale data over time is particularly challenging. However, if one could somehow remove the effect of known factors (much like economic indicators are seasonally adjusted) the underlying trends may become visible. Accounting for known factors in transport in surface runoff of the pesticide atrazine, routinely applied to corn and sorghum, was the objective of an index developed in this research. This index accounted for watershed area, the area in the watershed treated with atrazine, the timing of field operations, the timing of runoff, and the dissipation of atrazine at the edge of a field. It performed beyond expectations, accounting for more than 85 per cent of the variation seen in 4 years of atrazine transport out of 8 watersheds ranging in area from 74 to 1182 sq km (29 to 461 sq mi) and from 4 to 23 percent of the area planted to corn or sorghum. Using the index to help detect trends should benefit both regulatory agencies and producer interests interpret observations in many areas where atrazine remains under examination. A more immediate benefit is that, because the index performed well across a number of watersheds, it can be argued strongly that the watersheds in the surface runoff-prone claypan region behave in much the same way for atrazine transport, which has important implications for transferring results to unmonitored watersheds.