Sustainable Growth & the Future of Forested Watersheds

Mixed Land Use Watersheds in the Piedmont Vulnerable

urban skyline
The Piedmont Region of the Southeast is growing quickly. Photo by James Willamoor.

Forests provide high quality and dependable supplies of surface water. More than 19 million people in the Southeast get at least some of their drinking water from national forests, as U.S. Forest Service research revealed.

However, most forest land in the Southeast U.S. is privately owned. Such land could be converted to other uses in the future.

“Some models project that urban land uses in parts of the Southeast will double by 2060,” says James Vose, a project leader at the SRS Center for Integrated Forest Science.

“Average annual temperatures are projected to increase by 2 to 4 degrees Celsius,” says Vose. “Precipitation projections are much more uncertain, but many of the models project greater variability; that is, dry years become drier and wet years become wetter.”

Understanding how such changes could impact water supplies has been a major challenge.

Vose is part of an interdisciplinary research team devoted to untangling the potential impacts. The team recently published initial findings in the journal Ecohydrology. Katherine Martin, a research fellow with the Center and an assistant professor at North Carolina State University, was the senior author of the study.

The scientists modeled three watersheds in the Yadkin–Pee Dee River Basin, which stretches from the mountains of Virginia to the coast of South Carolina. The basin, which drains over 7,000 square miles, includes major cities, including much of the Charlotte metropolitan area.

Upper Dill Falls, NC
Forested watersheds provide clean drinking water to millions of people. Photo by Sarah Farmer.

The Yadkin-Pee Dee River Basin is part of the Piedmont region, which is growing quickly. Over the next 40 years, up to 20 percent of Piedmont forests could be replaced with varying intensities of development. The changes could have profound impacts on water supplies.

“Research team members John Coulston and David Wear developed a new model for projecting future land use patterns at small spatial scales,” says Vose. “The model allows us to understand how fine-scale land use patterns impact hydrologic processes and provided information at scales small enough to be useful for city planners and other decision makers.”

“The full range of possible futures is difficult to capture in one analysis,” says Vose. “However, our approach that combines future land use scenarios and climate projections with a detailed ecohydrological model can show where the impacts may be greatest.”

The scientists modeled watersheds that represented a range of land uses, from urban to heavily forested.

In the future scenario where timber prices are low, the land use model projected that more forest would be converted to other land uses. In scenarios where timber prices are higher, the results suggest forest loss would still occur, but at a slower pace.

“In every land use scenario we tested, we found that losing forests will affect water resources,” says Vose. “We found that mixed-use watersheds were especially vulnerable to forest loss”

When impervious surfaces replace forests, rain does not seep into the soil – it runs off the surface and into storm drains and streambeds. High flows can become more common.

southeastern U.S.
The model can help city planners see where forest losses could have the most impact. Image courtesy of NASA.

“Increasing drought will reduce water availability in some years,” says Vose. “However, as forest cover is reduced, water managers should also anticipate periods much higher flows in extremely wet years.”

Water managers and city planners can use the model to see where forest loss might have the greatest impact. Knowing which forests are vulnerable can help guide management and policy responses.

“Sustainable growth is possible,” says Vose. “One option is to focus on conserving forest land uses in areas that are the most vulnerable to forest loss and climate change.”

Read the full text of the article.

For more information, email James Vose at jvose@fs.fed.us.

Access the latest publications by SRS scientists.

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