Study evaluates runoff patterns at utility-scale solar sites

A new Preliminary Report on Runoff Characteristics from Utility-Scale Solar Sites, submitted to the Virginia Department of Environmental Quality (VDEQ), offers some of the clearest field data to date on how large solar installations influence stormwater runoff. The study, conducted by researchers at Virginia Tech, provides early findings that may help regulators, designers, and local governments refine stormwater expectations for rapidly expanding solar development.

According to the report , researchers monitored multiple storm events across two Virginia solar sites in 2025, capturing rainfall, infiltration, soil moisture and runoff behavior under vegetated solar arrays. The goal was to quantify how much rainfall becomes runoff and under what conditions.

The team found that runoff occurred only when rainfall intensity exceeded the soil’s infiltration capacity — approximately 5 mm/hr. For smaller storms, most rainfall infiltrated.

For individual storm events:

• A 25 mm rainfall event generated roughly 2 mm of runoff.

• A 47 mm event produced about 4 mm of runoff.

• A 52 mm storm resulted in approximately 8 mm of runoff, the highest measured in the study.

Runoff coefficients were low across events — 0.02 for the May storm and 0.03 for the August storm — indicating that the majority of rainfall infiltrated rather than becoming stormwater discharge.

Researchers also noted that vegetation density, soil structure, and the spacing of solar panels influenced infiltration, and that “hydrophobicity” in soil beneath panels may temporarily reduce infiltration during heavier storms. However, vegetation cover in the study areas remained high and relatively uniform, contributing to stable runoff behavior.

The findings support the idea that, when properly vegetated and managed, utility-scale solar installations can maintain relatively low runoff rates compared to other forms of land development. The report emphasizes that continued monitoring is needed to validate longer-term effects of vegetation maturity, maintenance practices, and seasonal variability.

The full Preliminary Report on Runoff Characteristics from Utility-Scale Solar Sites is available through VDEQ and provides detailed data intended to inform stormwater modeling, design assumptions, and regulatory decisions as solar development continues to accelerate across the region.