Land Use and Land Cover in the Chesapeake Bay
The Relevance of Land Cover and Land Use
The Chesapeake Bay is the largest estuary in North America, encompassing a 168,000 sq. kilometer watershed that covers parts of six states and the District of Columbia. The mixing of salt and freshwater in the Bay creates a diverse and complex ecosystem, which historically has supported thousands of migratory and resident species, including oysters, blue crabs, shad, herring, and waterfowl. The numbers of fish and wildlife that the Bay once supported has diminished drastically due in part to overharvesting and disease, but pollution from urban runoff, agriculture, and inadequate sewage treatment has caused a serious decline in water quality.
Restoration of the Bay has been the focus of a two-decade regional partnership of local, state, and federal agencies, including a network of scientists, politicians and political activists interacting through various committees, working groups, and advisory panels within the Chesapeake Bay Program. The overall health of the Bay has not declined since the restoration was initiated in 1983, but many of the advances have been offset by the pressure of increasing population and exurban sprawl across the watershed. The needs of the Chesapeake Bay Program are many, but the greatest is accurate information on land cover and land use change, primarily to assess the implications for water quality, examine various restoration scenarios, and to calibrate spatial models of the urbanization process. The work of the Woods Hole Research Center focuses on developing that information for use in this key ecosystem.
One of the successes achieved in the effort to improve the water quality in the Chesapeake Bay has been the reduction of pollutants from point sources, particularly municipal waste water treatment facilities, but more recently including industrial animal farming operations. The other significant source of material fluxes into the Chesapeake Bay are from non-point sources, or those that are distributed across the landscape and those closely associated with land use.
Approximately 40% of excess nutrient pollutants are introduced to the Bay through agricultural practices. Accurate estimates of land in agricultural uses are important for assessing impacts on water quality. Other substantial non-point sources of nutrients originate in urban or suburban areas, including the transportation network, through storm and water runoff. Accurate tracking of these developed areas, which are dominated by impervious surfaces (such as buildings, roads, houses, driveways, parking lots and the like), is required at a relatively fine spatial resolution. Impervious surfaces also modify stream hydrology and can result in substantially increased sediment transport.
Some of the adverse effects of impervious and agricultural areas can be mitigated by tree cover and stream side vegetation buffers, which reduce the force of overland flows, uptake excess nutrients, maintain stream bank integrity, and provide shade that reduces solar warming of waterways. Accurate maps of forest cover and riparian tree canopy density are needed for the ecosystem models that are integral to the restoration efforts in the Bay. When used together, maps of land cover types, impervious surface area, and tree cover provide the fundamental variables needed to assess the impacts they impart, the ecological functions they serve, or both.