Stormwater management practices, permitting, and regulations at solar sites often do not account for the unique characteristics of large-scale solar installations, according to a new Department of Energy (DOE) funded study.

Land change due to solar PV installations can differ considerably from residential and commercial development regulated by federal Clean Water Act stormwater standards. 

The study by the Great Plains Institute, the National Renewable Energy Laboratory, the University of Minnesota, and Fresh Energy found that key drivers of stormwater runoff were not included in regulation. Those factors include soil compaction, soil depth, ground cover establishment, and the distance between solar arrays.

The “Best Practices: Photovoltaic Stormwater Management Research and Testing (PV-SMaRT)” analysis, detailed and documented solar development characteristics that affect water quality. 

The analysis offered best practices for four parts of solar development that typically have a large impact on managing stormwater and improving water quality outcomes:

• Managing soil compaction and bulk density, which modeling results indicate is the most important consideration in determining stormwater runoff. Looser soils allow much greater water infiltration and allow improved and more rapid vegetation growth, which can further help maintain lower bulk densities.
• Understanding the importance of soil depth on the site. Deep soil allows more water to infiltrate the ground, while shallow soil can lead to more runoff.
• Installing, establishing, and maintaining appropriate vegetated ground cover between and under the arrays to facilitate water infiltration in the ground. That includes using or accounting for ground cover types, including bare dirt, turf grass, and native prairie.
• Designing the distance between arrays on the solar farm to increase infiltration. If there is more distance between arrays, there is more soil to infiltrate water. This could make a difference for sites with shallow soil depth that could be compensated for by widening distances between arrays.

The PV-SMaRT research findings were intended to demonstrate that additional stormwater infrastructure may not be needed under some site conditions when using good site design considerations, even for 100-year frequency design storms. When best practices are not followed, significant additional stormwater management may be needed, the report said.

The research findings also suggested that runoff estimates are, on average, 38% lower using PV-SMaRT methods relative to the Natural Resources Conservation Service runoff curve number method for a 100-year storm. Overestimating runoff can have a cost impact on solar projects, the report said, which would need more land for stormwater basins or for additional best management practices.

As part of the PV-SMaRT program, the University of Minnesota developed a stormwater runoff calculator to help users estimate runoff amounts for specific types of conditions. The calculator also comes with a user manual to guide users on its correct use.