Chesapeake Bay water quality has significantly declined over the past century due to increasing export of nitrogen (N), phosphorus (P), and sediment from the watershed. A major source of N, P, and sediment in rural watersheds is agriculture. Many best management practices (BMPs) have been established to reduce N, P, and sediment export from agricultural areas, and these BMPs have been shown to be effective at smaller plot scales. However, studies evaluating the ability of BMPs to improve water quality at the larger sub-watershed scale are limiting, and those that have been conducted had mixed success. This study assesses how the theoretical application of four different BMPs (riparian buffers, controlled drainage structures, variable rate N application, and cover crop) impacts stream N. N reductions due to different BMP scenarios were calculated using both Chesapeake Bay Program model BMP efficiencies and literature values. Our goal is to determine how many BMPs need to be applied to these watersheds to produce detectable decreases in stream N and to achieve Chesapeake Bay N water quality goals.
Results/Conclusions
Preliminary results suggest that theoretical, high-density application of BMPs can reduce N concentrations discharging in base flow from our experimental watersheds. Theoretical, saturated application of riparian buffers, controlled drainage structures, variable rate N application, and cover crops resulted in N concentration reductions at the South Forge watershed outlet of 20.3, 8.7, 12.2, and 26.5%, respectively. Preliminary results suggest that application of cover crops after both soybeans and corn have the largest impact on water quality within this watershed, but results in other watersheds will depend on the pre-simulation BMP coverage and the chosen BMP efficiencies, which vary significantly in the literature. Additionally, many farmers on the Eastern Shore of Maryland already use cover crops, and some watersheds have no suitable locations to install controlled drainage structures. Some watersheds also already have the majority of streams in riparian buffer, leaving little opportunity to enhance coverage. The effectiveness of enhanced BMP adoption significantly depends on the current BMP coverage, the density of applications, and watershed characteristics such as soil type. The enhanced application of BMPs throughout small watersheds can theoretically improve water quality, but varying combinations of BMPs based on watershed characteristics may prove more effective than the current voluntary approach.