For the last several years the Chesapeake Bay Program Office of the U.S. Environmental Protection Agency (EPA) has worked with local governments and their partners in all 206 counties within the Chesapeake Bay watershed across six states and the District of Columbia. All the Chesapeake Bay watershed counties and major municipalities have gathered together information on local land cover, land use, parcel and zoning data and converted it to a consistent format so that it could be accessed and used by the EPA.
Thanks to the hard work of groups like our own Prince William Soil and Water Conservation District and the Prince William County Government local land use data was collected from over 80% of counties. In parallel with these activities, the counties and municipalities funded the development of new high-resolution data on land cover—such as impervious surfaces, tree cover and water—for the entire watershed. This work was carried out by the Chesapeake Conservancy, the University of Vermont and World View Solutions, mapped out land cover across more than 80,000 square miles at a one-square-meter resolution. This is amazing resolution. This land cover data was then combined with the information provided by the various local governments and agencies to produce a detailed land use dataset for each county.
Now the EPA has used the high-resolution mapping of land use to update and improve the EPA Chesapeake Bay Program’s Chesapeake Bay Watershed Model, used to measure success against the EPA mandated Chesapeake Bay restoration activities and support local, state and regional decision making across the region. The latest version of this model, Phase 6, is currently under review. The EPA mandated a contamination limit for nutrient contamination and sediment to all the states in the Chesapeake Bay Watershed and Washington DC. The EPA set a total limit for the entire watershed of 185.9 million pounds of nitrogen, 12.5 million pounds of phosphorus and 6.45 billion pounds of sediment per year which was a 25% reduction in nitrogen, 24% reduction in phosphorus and 20 % reduction in sediment from the 2011 levels. The pollution limits were then partitioned to the various states and river basins and counties based on the Chesapeake Bay computer model and monitoring data. The problem with the first versions of the model was the land use data and impervious ground cover data was not consistent across different parts of the model.
The pollution limits were created by a series of models of the Chesapeake Bay Watershed. These computer models are mathematical representations of the real world that estimate environmental events and conditions. The models are at best imperfect, but they are nonetheless the best tool available to view the 80,000 square miles of the watershed. The Chesapeake Bay and its watershed are so large and complex, that scientists and regulators rely on computer models for critical information about the ecosystem’s characteristics and health, then use the model to assess the impact of various environmental mitigations to reduce pollution.
The earlier versions of the model had used approximately 675,917 acres for the impervious surface data and 1,885,915 acres for the pervious surface data in Virginia. A review of the EPA’s own data found that there were 1,569,377 impervious acres and 3,442,346 pervious acres in the urban areas in the Virginia segments of the model that includes all the paved and landscaped areas of suburbia. Between the 1990 census and the 2010 census when the model was developed the population of Virginia grew from 6.2 million people to 8.0 million people. The bulk of that growth took place in the urban and suburban centers of the Chesapeake Bay watershed. This served to distort the model and resulted in the under reporting of pollution from impervious surfaces (like roads, buildings and parking lots). Pollutions loads for nitrogen, phosphorus and sediment in the urban areas are calculated using a constant pounds/acre/year for impervious acres as a fixed input, and the pervious load is based on total fertilizer sales data less the impervious load. The under reported acres of housing and roadways distorted the agricultural contribution to the pollution by increasing it. Hopefully in the latest version of the model, that has now been corrected.
The datasets will be made available free-of-charge to local governments and the public over the next month or so. In addition, local governments will make available the data they collected on past land cover and land use over the last 30 years, as well as map overlays with geographic coverages of federal lands, sewer service areas, regulated stormwater areas and combined sewer overflow areas within each county. At the moment, Prince William and the rest of the Virginia counties are all still pending, but should be ready for viewing shortly at http://chesapeake.usgs.gov/phase6/ also the beta version of the Chesapeake Bay Model can be viewed at this link.