The accompanying river watershed pages—Susquehanna, Potomac, James, Eastern Shore, Western Shore, Rappahannock, York & Patuxent—present model estimates of the amounts of nitrogen and phosphorus that will reach the Bay when actions under way are fully effective.

To understand these figures, readers need to understand three basic concepts: modeling, nutrient sources and best management practices (BMPs).

Figures and charts on these pages show model-estimated changes from 1985, the baseline for measuring nutrient reductions.

Charts also show the level of implementation since 1985 for the six most effective nutrient control BMPs identified in tributary strategies for each river basin, (not counting wastewater treatment).

The Model

The Watershed Model is a tool used to estimate the amount of nutrients and sediment reaching the Bay. Like a giant accounting program, the model predicts the amount of nutrients expected to run off different land uses (nonpoint sources), then subtracts from that number the nutrient and sediment reductions expected from the implementation of various BMPs. (Point sources, though, are actual figures from discharge monitoring.)

There are some important caveats to model estimates:

  • The model estimates the amount of nitrogen and phosphorus that would be expected to reach the Bay under normal river flow conditions. Wet years flush more nutrients off the landscape, while dry years contribute fewer nutrients. These are not actual figures for 2005, but rather estimates of what nutrient loads would be under average rainfall conditions based on management actions taken through 2005.
  • The model does not address “lag times.” Half of the nonpoint source nitrogen reaches streams through groundwater. On average, it takes about 10 years for groundwater to reach streams, although the range is from hours to decades. As a result of this delay, nearly half of the nonpoint source, nitrogen control actions taken today will not impact the Bay for years. Likewise, much of the phosphorus is bound to sediment, which may take decades to reach the Bay.
  • The model assumes all reported nutrient control practices are occurring and being properly maintained. In other words, it assumes that if a nutrient management plan is written, it is also being fully implemented. Surveys suggest that this is often not the case. It also assumes that agricultural and urban runoff control practices are maintained over time to retain their effectiveness.
  • Conversely, the model only includes nutrient control practices reported by the states. It is likely that some actions, particularly those not funded by government programs, are not fully reported.

It is best, therefore, to think of these numbers as estimates of nutrient reductions that will occur in the Bay at some time in the future under “average” rainfall conditions.

Nutrient Sources

Nutrients originate from a host of sources. Different types of land release different amounts of nutrients over the course of the year; forests tend to release the fewest nutrients per acre, while agricultural fields or areas with large amounts of paved surfaces can release the largest amounts of nutrients per acre. Not all sources are land uses: Point sources discharge directly into waterways, while others seep into groundwater from septic system drainfields.

The model estimates the amount of nutrients originating from numerous different land uses. But on the following pages—unlike past years—nutrient trends are presented for only two major categories: Point sources, which include all wastewater and industrial discharges, and nonpoint sources, which include all runoff sources and septic tank releases. The reason for the change is the Bay Program is in the middle of a major upgrade to its Watershed Model which, among other things, will dramatically revise (and substantially improve) land use classifications. As a result, figures presented this year would not be comparable to those in next year’s report.

Best Management Practices

Best management practices is a term that covers a variety of management actions that control nutrient and sediment runoff, from changes in the way farmers till their land to the construction of stormwater systems in cities.

The Bay Program has assigned predicted nutrient reduction values for more than three dozen best management practices. The level of reductions may vary based on factors such as the width of a buffer strip or other factors. These are some of the most common BMPs in tributary strategies:

  • Riparian Forest Buffers are linear wooded areas along rivers, streams and shorelines that filter nutrients, sediment and other pollutants from surface runoff while removing nitrogen from groundwater. The minimum width must be 35 feet.
  • Wetland Restoration includes activities that re-establish or create wetlands in agricultural or urban areas.
  • Riparian Grass Buffers are linear strips of grass or other nonwoody vegetation maintained between the edge of fields and a stream that help to filter nutrients and sediment. The minimum width must be 35 feet.
  • Tree Planting is replanting open land with the intent of producing forest-like conditions over time.
  • Carbon Sequestration is a permanent grass cover (such as switchgrass) which is maintained for a decade or longer to sequester carbon from the atmosphere. The resulting crop may also be sold as a biofuel.
  • Conservation Tillage involves planting and growing crops with minimal disturbance of the surface soil, and leaving some crop residue in place to minimize erosion and runoff.
  • Nutrient Management is the development of comprehensive plans that describe the optimum use of nutrients to minimize nutrient loss while maintaining yield.
  • Conservation Plans are a combination of agronomic, management and engineered practices aimed at protecting and improving soil productivity and water quality.
  • Cover Crops are planted shortly after the fall harvest to absorb nitrogen left in the soil that would otherwise escape into groundwater.
  • Pasture Grazing Management can include a variety of actions such as stream fencing with off-stream watering sites; establishing off-stream watering sites without fencing; or stream fencing, off-stream watering and rotational grazing.
  • Enhanced Nutrient Management applies nitrogen at a rate 15 percent less than recommended in a nutrient management plan.
  • Urban Nutrient Management involves reducing fertilizer on grass lawns and other urban areas.
  • Urban Stormwater Management includes a variety of stormwater controls such as wet ponds and wetlands, dry detention ponds, infiltration practices and other techniques.
  • Urban Erosion & Sediment Control includes practices aimed at reducing sediment runoff associated with land development activities, such as a silt fence or the use of vegetation or a slope drain.

Major Nutrient Sources

  • Forest includes any woodlot larger than 28 meters by 28 meters, including forests, parks and even trees in a highway median strip. A large part of forest nitrogen runoff is thought to stem from air pollution.
  • Agriculture includes all land related to agriculture: from cropland to pasture to confined animal operations.
  • Septic includes human wastes from developments not hooked into sewer systems. (Phosphorus from septic systems does not reach waterways.)
  • Urban includes suburban, urban, industrial and commercial land uses, as well as roads.
  • Point Sources are discharges delivered directly to streams from the end of a pipe, wastewater treatment plants and some industrial activity.
  • Mixed Open includes land with low levels of development and low population densities that is not in agricultural use. This is often considered land that is in transition from agriculture to either urban or forest.

Where’s the Air?

Air pollution contributes at least a quarter of the nitrogen entering the Bay, but it doesn’t show up in these pie charts. Atmospheric deposition is distributed in the model among the land uses it falls upon. About two thirds stem from nitrogen oxide emissions from fossil fuel combustion. The remaining third comes from ammonia emissions, mainly from farms.