By Phil Potter, Kennedy Jenks Consultants Honolulu
Elevated ground-mount solar photovoltaic (PV) facilities present a unique situation for stormwater management because they usually involve an impervious surface elevated above a pervious vegetated surface. In this article, we will discuss the unique hydrologic processes at these solar PV facilities and the associated stormwater permitting requirements in various states across the country.
Hydrologic Processes at Solar PV Facilities
Stormwater runoff from solar PV facilities is generated primarily from rain that falls on access roads, inverter pads, and solar PV panels themselves. Water that falls on solar PV panels runs down the panel to the dripline, and eventually falls to the underlying surface, potentially causing localized erosion and/or scour. The primary factors that influence the potential for erosion and/or scour are shown on Figure 1. Some of the water falling on solar PV panels will infiltrate and some may run-off downslope and eventually to a collection basin or off site.
There is some debate as to whether the solar PV panels themselves have a significant effect on runoff volumes, peak runoff or times to peak runoff. A 2011 study published by the American Society of Civil Engineers  found that solar PV panels themselves do not have a significant effect on these key stormwater characteristics. The study notes, however, that if the ground cover under the panels is gravel or bare ground, resulting from design decisions or lack of maintenance, the peak discharge may increase significantly. In addition, the study found that the kinetic energy of the sheetflow from the panels was greater than that of the rainfall, which could cause erosion at the base of the panels.
For more detailed information, the reader is directed to the Minnesota Pollution Control Agency’s calculator with detailed instructions for project proponents to estimate the hydrologic impacts of installing solar PV panels .
Approaches to Stormwater Permitting at Solar PV Facilities
Based on a brief internet search conducted in July 2017, the following state stormwater regulatory agencies have developed guidance or specific requirements for stormwater management at solar PV facilities. Most agencies do not count panels as impervious cover, reasoning that runoff can flow beneath the panel and infiltrate into the ground the same as it did before the panel was installed above it.
Maryland’s Department of the Environment guidance states that for the purposes of issuing a stormwater permit for a solar project, calculations relating to the impervious surface of the project must include only the foundation or base supporting the solar PV panel .Maryland guidance further suggests that developers consider the following factors :
• Vegetated area receiving runoff must be equal to or greater in length than the disconnected surface (e.g., width of the row of solar PV panels)
• Runoff must sheetflow onto and across vegetated areas to maintain the disconnection
• Disconnections should be located on gradual slopes (≤ 5%) to maintain sheetflow. Level spreaders, terraces, or berms may be used to maintain sheetflow conditions if the average slope is steeper than 5%. However, installations on slopes greater than 10% will require an engineered plan that ensures adequate treatment and the safe and non-erosive conveyance of runoff to the property line or downstream stormwater management practice.
• Construction vehicles and equipment should avoid areas used for disconnection during installation of the solar PV panels.
• Groundcover vegetation must be maintained in good condition in those areas receiving disconnected runoff. Areas receiving runoff should be protected from future compaction.
The New Jersey Department of Environmental Protection exempts solar PV panels in calculations of impervious cover for the purposes of stormwater permitting.
The Massachusetts Department of Environmental Protection has indicated that solar PV panels should not be considered impervious.  However, guidance from the Massachusetts Department of Energy Resources (DOER) suggests a site-specific approach to managing stormwater at a solar PV facility because “panels could have the effect of altering the volume, velocity, and discharge pattern of stormwater runoff” . Several large ground-mounted solar PV projects being installed in Massachusetts include additional stormwater basins or other best management practices (BMPs) as part of their design in order to comply with the Massachusetts stormwater standards.
The Pennsylvania Department of Environmental Protection considers solar PV panels to be pervious cover and does not require additional Post Construction Stormwater BMPs, provided the following guidelines are followed :
• Earth disturbance and grading activities must be minimized and natural vegetal cover must be preserved and/or restored.
• Vegetal cover must have 90% or better uniform coverage and must not be subject to chemical fertilization and herbicides/pesticides. A meadow condition is preferable, particularly for slopes between 5 and 10%. Mowed areas, where approvable, should be kept to a minimum of 4".
• Individual PV panels within an array must be arranged in a fashion that allows the passage of runoff between each module. If the width of the module exceeds 3 feet (i.e. there is inadequate spacing between modules), then BMPs such as infiltration trenches (min. 12" wide by 12" deep) or infiltration berms must be installed downgradient between each row. The panels must be arranged to allow the growth of vegetation beneath and between arrays.
• Ground-mounted solar PV panels must be supported with structures/foundations occupying a maximum of 5% of the total project area.
• Solar PV panels must be situated on mild slopes (10% max). If larger than 10% slopes are proposed, then BMPs such as infiltration trenches (min. 12" wide by 12" deep) or infiltration berms shall be installed downgradient between each row.
• The lowest vertical clearance of the solar PV array must be at an elevation of 10 feet or less from the ground, but is also at an adequate height to promote vegetative growth below the array.
North Carolina allows solar PV panels to be considered pervious if they are configured to promote sheetflow of stormwater from the panels and natural infiltration of stormwater into the ground beneath the panels. Other structures associated with the solar PV facility such as buildings, entrance roads, transformers, and footings are still considered impervious. 
The Minnesota Pollution Control
Agency (MPCA) allows for the use of a volume credit for solar PV facilities that are vegetated beneath and between panels. This excludes sites that have rock bases . The water quality volume calculation may be completed using the disconnected impervious credit method shown in the Solar Panel Calculator on the MPCA’s webpage. The disconnected impervious credit method uses an Excel spreadsheet to calculate 1) the total
water volume required credited and
2) the remaining water quality volume to be treated. Depending on site-specific conditions, solar PV facilities can expect a 50% - 85% reduction of required water quality volume. The remainder of the required water quality volume must be treated on site. 
Irrespective of state-specific permitting approaches, elevated ground-mount solar PV arrays may have the potential to alter the volume, velocity, and discharge pattern of stormwater runoff at a site during and after construction. According to MPCA, sites can expect a 15–50% increase in volume due to the installation of solar PV panels. Additionally, a solar PV development site stripped of vegetation may result in erosive stormwater flows. Project proponents are advised to carefully consider the impacts of this additional runoff on their operations and overall compliance with environmental regulations.