Land Use Changes in Chesapeake Bay Watershed

 This past summer the  Chesapeake Bay Program released an update to its High-Resolution Land Use/Land Cover Data and Change Data and, for the first time, Hyper-Resolution Hydrography Data that precisely identify the location, dimensions and connectivity of streams, ditches and other waterways.

The updated Land Use/Land Cover Data, allows us to look at the changes in land use and land cover from 2001-2019. This tool was designed to see how the land of the Chesapeake Bay watershed is being managed and how it’s changed over time, it can also be used to look at the smaller watersheds that makeup the larger Chesapeake Bay watershed. The data is available to everyone, including county and state planners and conservation groups. The data can be widely used to address community planning, conservation efforts, human safety planning around flooding and environmental restoration efforts across the Chesapeake Bay, Potomac River and the Occoquan Watersheds.

The hydraulic and tree cover data were produced by the University of Maryland, Baltimore County (UMBC) in collaboration with the Chesapeake Conservancy Conservation Innovation Center. The University of Vermont Spatial Analysis, Chesapeake Bay Program and the USDA Forest Service also participated in the project. The U.S. Geological Survey (USGS) helped secure funding for the work, and stewarded the data products through the USGS peer review process.

In general, developed lands in 2001 were more concentrated within towns and major metropolitan areas. Since 2001, developed and semi-developed lands have expanded around these urban areas, as well as extending into previously undeveloped regions. How this has impacted Prince William County can be seen in the map below where the gray area shows where the land was covered by impervious surfaces in 2001, and the red area shows impervious surfaces developed between 2002 and 2019.

Land development 2001-2019 in Loudoun and PW counties

The proportion of land in the Potomac watershed classified as urban rose from 10% in 1985 to 17% in 2021 as identified in the Chesapeake Assessment Scenario Tool. This is problematic. Land use changes that increase impervious cover more than 5-10% from roads, pavement and buildings does two things. It reduces the undeveloped area for rain and snow to seep into the ground and percolate into the groundwater and the impervious surfaces cause stormwater velocity to increase preventing water from having enough time to percolate into the earth, increasing storm flooding and preventing recharge of groundwater from occurring. 

Slowly, this can reduce natural water supply over time and this is the watershed for our drinking water supply. As groundwater levels fall, perennial steams that feed the rivers become ephemeral. The groundwater becomes disconnected from the surface water network. Once the area hydrology is destroyed by development, it cannot be restored. Though there have been a few attempts, mankind has not succeeded in restoring a watershed. 

The research shows that Prince William County and Loudoun County have lost nearly 5,400 acres of tree canopy to development over just seven years due to human development. The emerging threats to tree canopy in Virginia include; the construction of utility scale solar arrays, warehouses, data centers road expansions and electrical transmission lines. 

Prince William County had PW Forest Park as a significant portion of tree cover

from Chesapeake Tree Canopy Network

When the land use changes completely to something other than forest, pasture or agriculture, then you lose the infiltration ability of the land and maintain the health of the rivers, stream and the land itself. The data set did not capture the acceleration of development in our region since 2021. The biggest solar operations and data center campuses in Virginia were installed after the most recent mapping imagery was captured, currently under construction or still in development but approved. Landscape changes like these are considered permanent.