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.
Fecal deposits by grazing animals on pasturelands have the potential to leach nutrients to runoff during rainfall events. Unlike croplands, grazing systems such as pasturelands or rangelands have little opportunity to ameliorate nutrient runoff through in-field or edge-of-field management practices. Thus, we investigated the amounts and concentrations of nutrients in overland flow from simulated grazing lands. Two grazing management scenarios were simulated: continuous grazing represented by two sparsely vegetated (SV) plots and rotational grazing represented by two densely vegetated (DV) plots. In addition, there were two control plots. The plots were treated with standard cowpats and rainfall was simulated until overland flow at the edge of the plots reached steady-state. Higher runoff was observed from DV plots (9.97 mm) than SV plots (7.05 mm), but the average total suspended solids concentration in runoff from SV plots was approximately 17 times the concentration observed in runoff from the DV plots. The average total phosphorus (TP) concentrations were highest in plots simulating continuous grazing (5.91 mg L(-1)). In both DV and SV plots at least 83% of the TP was found to be in the dissolved form. The average total Kjeldhal nitrogen (TKN) and total nitrogen concentrations observed in runoff samples from SV plots were 1.25 and 1.46 mg L(-1), respectively. Organic nitrogen comprised 95% of the TKN observed in runoff samples from SV plots. The SV plots have relatively higher loads for those nutrients in the particle associated form compared to DV plots, whereas DV plots had higher loads for those nutrients in the dissolved form. Grazing lands without any additional manure applications were found to release nutrients in high levels and vegetation did not show any effect on removing dissolved nutrients from runoff. These results are useful to inform selection of appropriate management practices to reduce nutrient transport to surface waters in watersheds dominated by grazed lands.
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