Stormwater runoff has become part of a growing, contentious debate around solar energy development in Virginia.
Some residents and officials in rural areas complain of flooding and pollution around large solar sites. Developers argue that there are already requirements in place to mitigate the problem.
Virginia Tech is in the midst of an effort to collect on-the-ground data to help inform state leaders how to move forward.
The Department of Environmental Quality gave the school $6 million for a six-year study. About two years in, the research team recently published initial findings.
“What we found is, these areas that have solar development happening tend to produce runoff more quickly,” said Ryan Stewart, associate professor at Virginia Tech’s School of Plant and Environmental Sciences. “In many cases, though not all cases, we tend to see higher peak flows coming off the solar areas.”
Stormwater runoff happens when rain falls on impervious surfaces, such as roadways and parking lots, which do not absorb water. That can cause flooding, contribute to erosion and allow pollutants to wash into waterways.
Utility-scale solar projects in Virginia are often built on former farmland, which absorbs water. But glass panels on top allow water to run off.
What happens next “is kind of an open question,” Stewart said.
Some people believe that when the water falls from below the panels, it keeps flowing, eventually running off the site. Others argue that once the water drips off, it infiltrates into the soil beneath adjoining panels downhill.
His team measured runoff at three utility-scale solar sites, one in Surry County and two in the Piedmont region, each spanning hundreds of acres.
Researchers installed metal structures, called flumes, in the channels lining the solar sites to monitor water flow during storms.
The devices are hooked up to data loggers with cellular connections that upload data to the cloud every 30 minutes, along with rainfall data from local weather stations.
For comparison, the group measured runoff at reference points around the solar sites, such as grass or cropland, to represent their previous land use.
Between January 2024 and October 2025, the group recorded between 39 and 76 storms at each site, most with less than an inch of rain accumulation.
The results showed that the spots within solar arrays often saw “rapid, and at times substantial,” amounts of surface runoff.
“I think that points out that solar development is having an effect on hydrology,” Stewart said. “We can't just assume that the landscape is how it was before the solar construction.”
Last year, Virginia Tech published research on how to best manage land while building and decommissioning utility-scale solar sites. The paper states such facilities could impact 350,000 acres of the state’s agricultural and forested landscapes by 2045.
Recommendations included using conservative estimates for potential runoff, using sensors to trigger panels to move into vertical positions during heavy rainfall and minimizing the removal of topsoil.
Solar developers are required to address stormwater runoff, often using constructed ditches or retention ponds to store water, which then evaporates or slowly trickles into soil and streams.
Much of the regulatory debate comes down to a technical definition. The state previously considered only the foundations of solar panels to be impervious surfaces.
But Gov. Glenn Youngkin’s administration rolled out changes to classify panels as 100% impervious, saying stronger standards were needed to stem pollution in the heavily regulated Chesapeake Bay watershed.
The change “has caused a lot of consternation among some of the developers who think that's overkill,” Stewart said. “That's something we hope to get a better handle on through this project: What's actually happening with water that's coming off the panels.”
The Virginia Tech researchers plan to gather more information in the remaining years of the project, such as expanding to six sites, figuring out what the runoff contains and any changes to the soil itself.
Stewart said he’s mindful of the “strong interest on both sides.”
“We're just trying to collect information in the best way we can and analyze it in a transparent way,” he said. “So that everybody involved can take a look and see this is actually what's going on, versus what we assume or hope is happening.”
