Nitrogen pollution causes environmental, economic, and human health damages mostly due to agriculture fertilizer runoff based on government monitoring. Spatial models for nitrogen pollution have been created by hydrologists and economists to form hydrologic-economic models (tools to analyze costs and benefits of water quality in rivers and drinking water) at regional and field scales that determine the optimal spending to reduce social costs. Economists argue for a combination of the two scales to achieve the highest pollution reduction because targeting a highly polluted region will reduce total pollution, and targeting highly polluted fields in these areas will achieve the most reduction per field.
Minnesota’s Nitrogen Policy focuses specifically on regional scales because the focus is on reducing total pollution. However, if we can reduce pollution at a higher rate on the same number of fields by targeting more polluted fields, then total pollution will be reduced more by the field-scale model. This approach can reduce pollution at a much higher rate for the same price, saving ecosystems and water-dependent economies, so I tested the two models against each other. I use regional outputs from the SPAtially Referenced Regression On Watershed attributes (SPARROW) geodatabase and field-scale outputs from the Agriculture Conservation Practice Framework (ACPF) geodatabase to test if there is a significant increase in the Nitrogen (N) reduction rate per field in the field-scale model. Using the ACPF, I conducted a benefit/cost analysis with N reduction as the benefit and the cost in dollars for the solutions used to reduce nutrients by that much. I find that the field-scale model can significantly increase N reduction / dollar cost compared to the regional model by targeting fields with high runoff risk. However, in the test watershed, the field-scale model reduces total N in the watershed to a lesser degree than regional targeting because fewer fields are selected for best management practices. Policymakers and businesses who begin using field-scale targeting models to implement Best Management Practices (BMPs) should consider if there will be a significant increase in N reduction / dollar cost by individual watersheds because some watersheds may benefit more from field-scale targeting than others based on number of fields with high runoff risk.
Nitrogen pollution poses an existential threat to fish and wildlife and economic risks to US drinking water and recreation. Ten percent of US recreational fishing is directly impaired every year due to excess agricultural nutrients sourced into the Mississippi River by tributaries in the Midwest (Petrolia & Gowda). Nutrient pollution directly damages the economy upstream as coldwater wildlife is unable to live in eutrophic (excess nutrients and algal growth) rivers and lakes near the farms (Jacobson et al.). Two techniques that Midwest states use to reduce pollution include targeted practices on fields with the largest incremental pollution and non-target practices that apply a strategy to an entire catchment (landscape where all water pools together), watersheds (larger basins and catchments), or state.
In attempting to design better pollution reduction, economists developed large regional models to cover broad non-targeted areas on all available land. The works of Rabotyagov et al. (2010) sought to reduce nutrient pollution on a massive scale, configuring various best management practices along the Mississippi River and its tributaries and then configuring them into two options. The first option reduced phosphorus runoff at higher rates but increased nitrate runoff. They recommend the second option which uses grassland wetlands, fertilizer reduction, and land retirement together as the most cost-effective strategy (Rabotyagov et al.). While this strategy is only able to reduce 30 percent of nitrate, a combination of this regional strategy and a field-scale strategy could produce the most substantial abatement of nearly 40 percent.