Stormwater Management in Solar Farms

The rapid expansion of solar farms across Pennsylvania and the broader United States has raised a practical and often misunderstood question: what happens to water when fields are covered with solar panels? At first glance, the concern seems obvious. Solar panels are impervious surfaces, and conventional thinking suggests that imperviousness leads to increased runoff, erosion, and downstream impacts. But recent research conducted at Penn State challenges the idea that solar farms behave like traditional developed surfaces, such as parking lots or rooftops (Yavari et al., 2024). Instead, these systems create a far more complex and spatially variable hydrologic response, one that can be managed effectively when properly understood and designed.

Hydrology of a Solar Farm

Field observations from solar farm studies reveal that they do not simply generate runoff uniformly across the site. Rather, they redistribute rainfall in distinct patterns. Water that lands on panels is intercepted and concentrated along the panel edges, creating localized zones of higher moisture. Beneath the panels themselves (underpanel), conditions tend to be drier due to reduced direct rainfall. Between panel rows (interspace), vegetated areas act as zones of infiltration and storage.

Measurements from Penn State’s study at two solar farm sites in Centre County, PA with fixed-tilt panels showed that soil moisture beneath panels can be approximately 25% lower than reference land use, while zones along panel driplines can experience about 19% higher moisture. In addition to precipitation, evapotranspiration (ET, the sum of soil evaporation and plant transpiration) was found to play a key role in driving hydrological processes in solar farms, especially in interspace and underpanel locations. The study revealed lower ET under the panels, with potential ET about 37-67% lower in summer months. Vegetation height in the interspace was similar to that observed at reference locations without the panels and, in some cases, even taller. The soil moisture content measured at multiple locations on the study site validated the runoff patterns in solar farms, with underpanel zones remaining relatively drier and driplines showing increased wetness. The average soil moisture at the dripline was 18-21% higher than the reference, whereas the underpanel sections showed 20-29% lower soil moisture. These differences fundamentally change how water moves across the site. The implication is straightforward but often overlooked: solar farms are not uniformly impervious systems. They are heterogeneous landscapes where runoff generation depends on how water is routed, concentrated, and absorbed across different zones.

While the Penn State study provides valuable field-based evidence from solar farms in central Pennsylvania, stormwater responses can vary depending on local conditions such as soil type, slope, climate, vegetation cover, and panel configuration. Findings from other regions support the importance of site-specific design. For example, research conducted as part of the Photovoltaic Stormwater Management Research and Testing

(PV-SMaRT) project combined field monitoring and hydrologic modeling at multiple solar farm sites across the United States and found that vegetation establishment, soil conditions, and rainfall characteristics strongly influenced runoff generation and infiltration patterns (Mulla et al., 2024). Their work demonstrated that vegetation established beneath and between solar panels can substantially reduce runoff and erosion while promoting infiltration. The study also showed that hydrologic responses varied among sites depending on soil type, rainfall intensity, and vegetation characteristics, highlighting that stormwater management practices may need to be tailored to local conditions rather than applying a single design approach everywhere. Collectively, these studies suggest that while the fundamental processes of rainfall redistribution and concentrated flow near panel driplines are common across solar farms, the magnitude of stormwater impacts and the level of management needed can vary considerably from site to site.

Do Solar Farms Increase Runoff?

The answer is not a simple yes or no. For smaller rainfall events, studies indicate little to no increase in runoff compared to pre-development conditions. During larger storms, there is potential for increased peak flows, particularly if water concentrated along panel edges is not adequately managed. Additionally, sites with steeper slopes, poorly draining soils, or sparse vegetation can be more susceptible to runoff. Field evidence from operational sites in Pennsylvania suggests that when vegetation is well established and appropriate stormwater controls are in place, solar farms do not exhibit widespread erosion or runoff problems. The risk is not inherent to solar infrastructure itself, but to how the system is designed and managed.

In other words, solar farms can either amplify or mitigate hydrologic impacts depending on the decisions made during construction and maintenance. This conclusion is consistent with recommendations from a recent Chesapeake Bay Program Scientific and Technical Advisory Committee (STAC) workshop, which highlighted the importance of vegetation management, soil health, and site-specific stormwater planning in minimizing environmental impacts from solar development (McPhillips et al., 2024). The workshop report concluded that properly designed solar facilities can maintain many of the hydrologic functions of undeveloped landscapes, particularly when perennial vegetation is established and soil disturbance is minimized. However, the workshop also identified important research gaps related to runoff calculations, managing erosion risk, and improving vegetation establishment and long-term management.

Design Determines Outcomes

What emerges from existing research is a consistent message: stormwater performance in solar farms is driven far more by site characteristics and land management than by the panels themselves. Vegetation plays a central role. Dense, well-maintained ground cover slows runoff, enhances infiltration, and stabilizes soil. Without it, even modest rainfall can lead to erosion, especially in concentrated flow zones near panel edges. Equally important is the condition of the soil. Construction activities that remove healthy topsoil or compact the soil can drastically reduce the land’s ability to absorb water, effectively converting otherwise permeable land into a runoff-generating surface.

Another critical concept is the idea of "impervious disconnection". Unlike traditional development, where runoff is rapidly conveyed through pipes and channels, solar farms can be designed so that water leaving panels is immediately dispersed across vegetated areas. This breaks up flow paths and allows infiltration to occur before water accumulates downslope. Structural practices such as infiltration trenches or basins still have a role, particularly on sloped sites or where soils have limited permeability. Solar panels that tilt or track throughout the day may also reduce the redistribution of rainfall and the resulting impacts on runoff.

Gaps and Future Research

The broader takeaway from this research is that solar farms should not be viewed simply as energy infrastructure, but as managed landscapes with distinct hydrologic behavior. Rather than functioning as uniformly pervious or impervious surfaces, they consist of spatially heterogeneous hydrologic zones whose function is strongly influenced by site design and management.

Despite recent advances in understanding solar farm hydrology and stormwater management, significant uncertainties remain regarding how findings from individual sites translate across different landscapes, climates, and management practices. Questions that engineers and land managers routinely face, such as how to size stormwater controls, how panel spacing influences runoff, or how slope interacts with flow concentration, are not yet answered with sufficient precision across the wide range of landscapes where solar farms are being developed. While field studies provide important observations, modeling studies have become increasingly important for design and for evaluating scenarios that cannot be easily tested in the field. Recent research at Penn State developed a hydrologic modeling framework that explicitly represents solar panels, driplines, and vegetated interspaces within stormwater modeling tools (Nair et al., 2024). Their work showed that solar farms redistribute rainfall across the landscape, creating wetter zones near panel edges and drier zones beneath panels, patterns that closely align with field observations. Such models provide an opportunity to evaluate alternative layouts, stormwater controls, and future climate scenarios. The PV-SMaRT project team has also developed a Solar Farm Runoff Calculator tool to help calculate runoff from sites planned for solar development, that can help engineers better plan stormwater management. As solar development continues to expand, integrating field monitoring, vegetation management, hydrologic modeling, and engineering design will be critical for continuing to refine science-based stormwater guidance.

Virtual Solar Array Tool

The Virtual Solar Array project, developed by a team at Penn State, aims to create a digital representation of ground-mounted solar arrays and their interactions with water, soil, and vegetation. It shows the various physical and landscape features, with a particular focus on practices that support sustainable landscape management when developing and operating a solar array. The tool is intended to be accessible to a wide range of users, including general public. Users can view images on-the-ground from real solar farms and explore best practices for factors such as panel spacing, site slope, vegetation establishment, soil management, and stormwater control practices.

Explore the tool online

References

Yavari, R., et al. (2024). Quantifying soil moisture and evapotranspiration heterogeneity within a solar farm: Implications for stormwater management. Journal of Hydrology.

Penn State News (2024). Solar farms with stormwater controls mitigate runoff, erosion, study finds.

Yavari, R., et al. (2022). Minimizing environmental impacts of solar farms: A review of current practices and future directions. Environmental Research: Infrastructure and Sustainability.

McPhillips, L., et al. (2024). Best Management Practices to Minimize Impacts of Solar Farms on Landscape Hydrology and Water Quality. STAC Publication Number 24-001, Edgewater, MD.

Mulla, D., et al. (2024). Measuring and modeling soil moisture and runoff at solar farms using a disconnected impervious surface approach. Vadose Zone Journal.

Nair, A.A., et al. (2024). A Framework to Model the Hydrology of Solar Farms Using EPA SWMM. Environ Model Assess 29, 91–100. doi.org/10.1007/s10666-023-09922-0

PV-SMaRT Solar Farm Runoff Calculator Version 3.1 (2023). Accessed on July 26, 2026.