Guest Commentary: High-res Chesapeake Bay watershed maps reveal surprising development increase

Sussex County tops list of loss of impervious cover; Kent appears, as well


Making the struggle to restore the Chesapeake Bay even tougher, watershed states have been clearing more forest for development and paving over more landscape than previously believed, new data shows.

A recently released analysis of high-resolution aerial imagery taken four years apart indicates the watershed has been losing more than 20,000 acres per year of pollution-fighting forest to development and adding more than 12,000 acres annually of runoff-inducing pavement and buildings.

Those are just two of the more notable findings in a federally funded project to map land-cover and land-use change across the bay watershed using aerial imagery, which has a resolution that is 30 times higher than the satellite imagery previously used for this purpose. The project was conducted by the nonprofit Chesapeake Conservancy in collaboration with the U.S. Geological Survey, the University of Vermont and federal-state Chesapeake Bay Program.

Those involved in gathering and parsing the data say it provides a much more precise picture of how the bay region’s landscape is being used and how quickly it is changing.

“It gives us insight of what is happening on the ground,” said Joel Dunn, the conservancy’s president and CEO. “We can look back and see what happened in incredible detail.”

Officials hope to use the information to keep closer track of bay states’ efforts to reach their pollution-reduction targets by their agreed-upon 2025 deadline. Much of the progress to date has come from upgrading wastewater-treatment plants across the six states and District of Columbia.

But what happens on and to the land plays a major role in how much nutrient and sediment pollution gets into the bay via rain and snow runoff. On that score, the cleanup effort continues to fall short.

For the past 30 years, the bay-restoration effort has relied on satellite imagery to identify land-cover and land-use in the watershed. But those orbiting eyes in the sky can only see with accuracy features on the landscape that are at least 30 meters across. That can miss low-density housing and narrow streamside tree buffers, among other things.

“Without that knowledge, we’re just stuck in this no man’s land of uncertainty,” said Peter Claggett, a USGS research geographer who coordinates the Bay Program’s Land Use Workgroup.

So with funding from the U.S. Environmental Protection Agency and USGS, project participants collected high-resolution aerial imagery of the region for the years 2013–14 and again for 2017–18. The surveys encompassed 99,000 square miles in 206 counties, including all of the land in counties just partially in the bay watershed.

Analysts then painstakingly categorized the land use, whether in agriculture, homes, businesses or left to nature, by consulting other data, including records on parcel boundaries, mining activity, landfills, golf courses, utility transmission lines and timber harvest permits.

Thus curated, the high-resolution imagery can indicate what’s on the land down to a 1-meter scale, giving a far more detailed picture.

It revealed, for instance, that pavement and buildings cover about 45% more of the region’s landscape than had been identified with the less precise satellite imagery, Claggett said. That’s significant because these impervious surfaces prevent rainfall from soaking into the ground and serve as a conduit for pollution-laden runoff into nearby streams and rivers.

The imagery also shows 11% more new development than had been seen by satellite observations, Claggett said.

The biggest land-use change between the two periods came from timber harvesting, with 175,000 acres of forest cut down across the watershed in the four-year span. That’s a long-lasting but not permanent change, assuming those acres are reforested over time.

But nearly 83,000 acres of forest were cleared for development. Some trees remained on about one-third of those acres, though the undergrowth in those wooded areas had been replaced by buildings, pavement or turfgrass. Another 43,000 acres of forest were cleared in agricultural areas, which analysts assume went into cropland or pasture.

The high-resolution imagery also picked up a net loss of tree canopy in developed areas. While many communities planted lots of new trees, there was an overall decline in tree cover of about 12,000 acres in cities, towns, suburbs and even rural areas. Bay states and Washington, D.C., have pledged to add 2,500 acres of tree canopy by 2025 in urban areas alone.

Nearly 51,000 acres of new buildings and pavement spread across the landscape during the four-year span, with nearly the same amount of new turfgrass observed. There had been more low-density development in the past, with two to three times as much turfgrass as impervious cover, Claggett noted. This recent change signifies more high-density development, he said, perhaps with more apartment buildings and warehouses.

Biggest increases in impervious cover, 2013–14 to 2017–18

  • Sussex County* — 3,313 acres.
  • Lancaster County, Pennsylvania — 2,424 acres.
  • Loudoun County, Virginia — 2,222 acres.
  • Chester County, Pennsylvania* — 2,002 acres.
  • York County, Pennsylvania — 1,770 acres.
  • Cumberland County, Pennsylvania — 1,763 acres.
  • Kent County* — 1,746 acres

*Only partly in the Chesapeake Bay watershed

As large as these shifts seem, they are relatively small when stacked up against overall land use. There are 34 million acres of forest across the region, for instance, and roughly 2.5 million acres of impervious surfaces, Claggett noted.

But land use is not uniform across the watershed, he added, nor are the changes. Much of the forestland is publicly owned or otherwise protected, Claggett said. So what’s being developed is a significant share of the vulnerable natural areas.

The new data can identify those hot spots, project leaders say, and help state and local officials and concerned residents respond. Toward that end, the project partners have created maps of land-cover and land-use changes for each of the watershed’s counties and plan to make them public.

“My hope is that these data will be used … to inform more strategic planning and conservation decisions,” Claggett said. He also hopes it will raise public awareness of what’s happening on the landscape and how that may or may not impact them locally.

The maps can help locate the most effective sites for restoring degraded streams or planting trees and identify environmentally sensitive lands in need of protection.

Such detailed analysis is not cheap. The project cost about $3 million over the last four years, according to the conservancy. Even as analysts continue to vet the existing imagery, aerial surveys from 2021–22 is being compiled for analysis.

The Chesapeake Conservancy’s Dunn said he envisions that such detailed information on the bay watershed’s landscape and its changing use will empower community groups, businesses and even individuals to take steps to improve environmental conditions in their own neighborhoods.

“This is conservation innovation in action,” he said. “You give people this killer data, and there’s all kinds of ways they can use it.”

Timothy B. Wheeler is the associate editor and senior writer for the Maryland-based Bay Journal, where this was first published.

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