Rainfall experiments found solar panels cut soil erosion beneath their surfaces by 27–63%, while concentrated dripping created new erosion hotspots along panel edges

Solar farms are spreading fast, showing up on hillsides, farmland, and open fields as countries push to cut carbon emissions. But what actually happens to the soil beneath all those panels? A rainfall experiment, published in the Journal of Hydrology, sought an answer, and the results are more complex than just good or bad. The study titled ‘How a photovoltaic panel impacts rainfall-runoff and soil erosion processes on slopes at the plot scale’ found that soil directly beneath a solar panel lost 27% to 63% less material to erosion than open, unshaded ground during simulated rainfall. Erosion was worse on the bottom of the panel where the runoff drips off in a narrow, concentrated stream.

How the experiment worked

Researchers prepared two identical plots of land using loess, a loose, wind-blown soil common to the Loess Plateau in China and prone to erosion. One plot was open to the air. The other had a four-panel solar array on top, tilted like real solar farms are built. Both plots were then subjected to simulated rainfalls of varying intensity, from light showers to downpours as heavy as a strong storm. Sensors measured how fast water ran off each plot and how much soil it carried with it.


Why erosion is lower beneath the panels

The solar-panel plot performed better in nearly all cases. According to the research, most of the soil erosion occurring due to rain is not caused by flowing water. Rather, it is due to raindrops. Raindrops hit the bare ground as if they were small hammers, which is referred to as splash erosion. They make the soil particles loose, causing them to be washed away by moving water. The solar panel acts as a roof and stops the raindrops from hitting the soil, hence keeping it intact. In addition, the time taken for rainwater to become runoff was longer on the solar-panel plot, particularly in heavy rain, because of a slight depression in the soil at the lower part of the plot, just beyond the panel.

The edge problem nobody expected

The panels do not stop rainwater from falling; they simply redirect it. All the water that falls on the surface of the panels eventually runs off from one side of the panels and reaches the ground in a concentrated line, which is similar to that of a drip line under a roof gutter. That concentrated dripping was the focus of a companion modeling study in Water Resources Research, which simulated erosion at a real-world solar farm site using a hydrological model, and found that this concentrated dripping turns an otherwise gentle rain shower into something more like a mini downpour along that narrow strip. Most of the panel's shaded soil is protected, but the thin strip at the edge gets hammered. In the researchers' modeled solar farm, soil erosion rates increased by roughly 21.4% to 74.84% during construction and 25.35% to 76.18% during operation, and runoff volume rose by 99.18% to 154.26% during operation, nearly doubling to two-and-a-half times the original amount, showing how a few edge hotspots can add up when scaling a single panel into hundreds of rows.

A second study backs up the pattern

A separate study, ‘The Photovoltaic Panel Array Inhibits Initial Rill Development and Soil Erosion During Rainfall-Runoff Processes in a Soil Slope,’ reached a similar conclusion, also in Water Resources Research. Researchers used the four-panel system and also found a similar pattern. There was a 39.7% to 64.1% reduction in soil erosion mass and a 38% to 52.5% decrease in peak erosion intensity compared to the bare slope. In this research, the researchers also observed that the formation of rills, the narrow channels that can be formed in the soil under the effect of rain, was significantly inhibited when the slope was covered with panels.

How this affects expanding solar farms

Solar energy is one of the fastest-growing energy sources on the planet, and increasingly large farms are being built on sloped or hilly land where erosion is already a concern. These results suggest that panels are not as damaging to soil as some feared, and may even help protect the soil in the areas they shade. But they also point to a real design problem hiding in plain sight: the edges. The researchers behind these studies suggest simple fixes, such as planting grass or ground cover directly beneath panel edges, or adding small drainage channels to slow and spread out dripline runoff, could prevent those hotspots from turning into long-term problems.

The takeaway

Solar panels are not simply good or bad for the soil underneath them. They protect the ground they shade, quietly shifting the erosion problem to a thin strip at their edges. As solar farms continue to expand across hillsides and open land, the edge strip may prove as important as the panel itself.