|Bednarz, Craig -|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2006
Publication Date: September 13, 2006
Citation: Potter, T.L., Truman, C.C., Bosch, D.D., Strickland, T.C., Franklin, D.H., Bednarz, C.W., Webster, T.M. 2006. Combined Effects of Constant Versus Variable Intensity Simulated Rainfall and Reduced Tillage Management on Cotton Preemergence Herbicide Runoff. Journal of Environmental Quality. 35:1894-1902. Interpretive Summary: Runoff may carry pesticides from treated fields into streams and rivers and adversely impact water quality. To help farmers minimize impacts there is a continuing need for studies comparing how different products behave under differing management practices. To do this, researchers frequently depend on rainfall simulation methodology. In most simulations rainfall is applied at a constant rate; however such studies may not accurately reflect pesticide runoff behavior. This is because natural rainfall patterns are rarely if ever constant. Implications were evaluated in field-based simulations in which runoff of two herbicides commonly used by cotton growers was compared by simulated storm intensity pattern: constant versus variable and tillage: strip versus conventional. The variable rainfall pattern was representative of spring storms in the southern Atlantic Coastal Plain of Georgia (USA) where the work was conducted. We found that intensity pattern affected runoff rates, but on an event basis total runoff differences were relatively small. Tillage and properties of the herbicides had much greater impact on runoff. Overall, study results suggest that a more realistic representation of pesticide runoff behavior may be obtained by using variable storm patterns in simulations but improvements in total runoff estimates will be small. Thus, critical conclusions regarding pesticide runoff potential derived from the two methods of simulating rainfall will likely be similar. This information adds confidence in results of the large number of pesticide runoff studies conducted using the traditional approach to rainfall simulation, i.e. constant rainfall pattern. Results can be used by regulatory agency scientists who assess risks of pesticide use based on runoff potential as well as extension agents, USDA-NRCS staff and others who make pesticide use recommendations based on runoff and other pesticide characteristics.
Technical Abstract: Rainfall simulations are widely used to evaluate pesticide runoff potential. Most are conducted at constant intensity, i.e. at a fixed rainfall rate. In contrast large within storm intensity variation in natural rainfall is common. To assess implications, field-based simulations were conducted. Runoff expressed as percent of active ingredient applied was evaluated stepwise by simulation pattern: constant versus variable; tillage: strip- (ST) versus conventional (CT); and pesticide transport characteristics: aqueous phase dominated (fluometuron) versus sediment associated (pendimethalin). The variable pattern was representative of spring storms in the southern Atlantic Coastal Plain of Georgia (USA) where the work was conducted. The herbicides are commonly used by cotton growers. For both compounds under both tillages, simulation intensity pattern strongly impacted runoff kinetics. However, event-based runoff totals were not significantly different with one exception, fluometuron runoff from CT plots. The variable yielded about 25% more than the constant intensity pattern (P=0.10). This was linked to fluometuron’s tendency to leach and because soil was freshly tilled and relatively dry when simulations were conducted. Tillage and herbicide transport characteristics had a greater impact on total runoff. CT plots delivered 8 to 10 fold more pendimethalin in runoff than ST plots. Mean differences were significant (P=0.05). This was also case when pendimethalin and fluometuron runoff from CT plots was compared. An opposite trend was observed on ST plots. Fluometuron was greater than pendimethalin runoff. Fluometuron runoff was also greater from ST than CT plots; but differences in response were not significant due to high variability in fluometuron ST plot runoff. The study indicates that use of constant versus variable intensity rainfall in simulations will not substantially impact runoff trends with the possible exception of relatively water soluble compounds like fluometuron under CT conditions.