Modeling Pesticide Runoff from Small Watersheds through Field-Scale Management Practices: Minnesota Watershed Case Study with Chlorpyrifos

Experimental studies of pesticide fate in surface runoff offer only a snapshot of the near semi-infinite parameter combinations that can and do occur in the environment, and mechanistic modeling is often used to supplement the often limited number of experimental observations. However, what has been lacking in pesticide surface runoff modeling is the impact of field-scale best management practices (BMPs) on the concentrations of pesticides found at the watershed outlet. A novel application of melding three agricultural models together was used to address field-scale BMPs and off-target pesticide environmental concentrations at the watershed scale resulting from agricultural surface runoff. These models were the pesticide root zone model [PRZM, an edge-of-field runoff and leaching model sanctioned by the US Environmental Protection Agency (USEPA)]; the United States Department of Agriculture-Agricultural Research Service watershed scale model, the soil and water assessment tool (SWAT); and the academic model, the vegetated filter strip model (VFSMOD). Watershed models such as SWAT, using high-resolution local input data, are capable of predicting watershed behavior but are limited when addressing field-scale BMPs. A unique method to approximate a small watershed as a linear combination of sub-basins and fields [hydrologic response units (HRUs)] is presented. Water, sediment, and pesticide runoff for each HRU are simulated using the USEPA field model PRZM. Daily edge-of-field PRZM predictions for pesticides in runoff water and eroded sediment are coupled with VFSMOD to address the effectiveness of a maintained vegetated filter strip (VFS) across the growing season in reducing pesticide loadings and water quality at the watershed outlet. Daily chlorpyrifos (CHP, insecticide) concentrations simulated for the Seven Mile Creek Watershed, MN, using the above modeling approach resulted in a spectrum of concentrations reported by the MN Department of Natural Resources. Simulated VFS effectiveness when used across all pesticide-treated fields ranged between 22% and 100% reductions in CHP mass across all runoff-producing events.