Reducing phosphorus and sediment loss in runoff from agricultural land

Phosphorus (P) and sediment losses in runoff from agricultural land are a serious environmental issue because of the impact on surface water quality and eutrophication. The main factors affecting the transport of P to surface waters are erosion and runoff. Non-application setbacks, perennial vegetative filters, reservoirs, terraces, reduced tillage, and other management practices are intended to reduce P and sediment loss from the watersheds. Four major hypotheses were tested in this study: (1) P and sediment loss are reduced when manure is not applied within setbacks around conduits to surface water; (2) Grass filters will result in reduced sediment and P loading with increased effectiveness as the area ratio of filter to drainage basin increases; 3) 15-20% of the watershed area is the source of more than 50% of P and sediment loading; and 4) Contaminant loading of Duck Creek can be reduced by 30-50% by increasing the use of conservation BMPs on 50% of the watershed land area. Research was conducted under rainfed agricultural systems in eastern Nebraska to address the following objectives: (1) to determine the effectiveness of non-application setbacks for manure from conduits to surface waters in reducing P and sediment in runoff; (2) to evaluate the effectiveness of grass filters in reducing runoff volume and loss of P and sediment; (3) to assess the impact of management practices on sediment and nutrient losses from the watershed using a watershed model; and (4) to evaluate different management scenarios required to achieve further targeted reductions in sediment and P loss to surface waters using a watershed model. The seven treatments of the setback study included no manure applied and nonapplication setbacks of 0, 5, 10, 20, 30, and 40 m around the riser with the setback distances limited on the terrace side by the top of terraces. Runoff sampling methods were compared and the effects of drainage basin characteristics determined. The effect of grass filters on sediment and P concentration was determined from four runoff events in 2006-2008 when nearly all plots had runoff. The grass filters also were evaluated for effect on runoff volume and contaminants load using total annual runoff and composite samples collected from 12 runoff events. In the watershed modeling study, the SWAT model was used to evaluate the effects of tillage, grass filter strips, and terraces on nutrient and sediment loss. Critical areas for these contaminants were identified in the watershed and implementation of management scenarios was simulated on targeted vulnerable areas to identify means of reducing nutrient and sediment loss to targeted levels. Three BMPs scenarios were simulated using SWAT model on the small sub-basin to present the impact of different scenarios on sediment reduction and illustrate the importance of BMPs placing and distribution at hydrologic response units