The annual drought is no longer a concern only for farmers in Southern Europe. Across Central Europe too, growers are facing insufficient water, or rainfall at the wrong time. Although this year brought a relatively typical summer, many farmers in Germany still saw harvests ruined. Sunshine and dry conditions prevailed in spring and early summer, while heavy rainfall arrived during the harvest itself.
More water under modules?
Agri-PV systems are said to offer protection against such uncertainties, with the promise that solar modules help regulate the soil's water balance. But do they actually deliver? Initial findings from a study led by Ulrike Feistel, Professor of Engineering Hydrology at Dresden University of Applied Sciences, and hydrologist Stefan Werisch of the Brandis Lysimeter Station, suggest they can. With support from energy supplier EnBW, the team measured changes in soil moisture beneath several solar parks in Saxony and Brandenburg, including an agri-PV installation in Pillnitz near Dresden.
The researchers hypothesised that soil beneath solar modules would experience greater water recharge than uncovered ground. In areas of the solar park without panels, the balance between rainfall and evaporation remains similar to that of open land. Under the modules, however, evaporation is reduced, allowing more moisture to remain in the soil.
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Rainwater, on the other hand, reaches the ground only on the eaves side of the modules, running down the panels and off the lower edge. In a narrow strip behind each row, the same amount of precipitation falls as on uncovered land, but these areas are shaded from the southern sun, which reduces evaporation and helps retain more moisture in the soil.
Soil moisture determined
That was the theory, but does it hold up in practice? To find out, the team led by Feistel and Werisch measured soil moisture at a depth of ten centimetres beneath solar parks in Boxberg, Pillnitz and Weesow, comparing the results with nearby reference plots. The findings were clear: while moisture levels in the reference areas rose quickly after rainfall, they dropped off just as fast. Under the solar modules, by contrast, soil moisture remained more stable. In Boxberg, the reference plot showed a daily moisture loss of 0.99 to 1.88 percent, while the ground beneath the modules lost only 0.57 to 1.39 percent per day. The effect was particularly pronounced over several consecutive dry days.
More water in dry periods
A similar pattern emerged in Pillnitz beneath the elevated agri-PV installation. Although less rainfall reached the soil directly under the modules, moisture levels still declined more slowly during dry spells. In April 2020, soil moisture under the system fell by just 0.35 percent per day, against 0.69 percent in the reference plot.
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In Weesow, measurements were taken in late October and early November 2021. Again, less water reached the soil directly beneath the modules on rainy days, but evaporation was significantly lower too. Under the array, soil moisture fell by just 0.02 percent per day, against 0.27 percent in the reference plot.
While the study did not assess how reduced evaporation and shading affect crop yields, particularly near the drip edges of the modules, it clearly shows that both photovoltaic and agri-PV systems significantly influence soil moisture. According to the researchers, lower evaporation combined with greater water retention during dry periods may help stabilise the water balance across the entire site.
Researchers at the University of Science and Technology of China in Hefei, Anhui Province, also examined how agri-PV systems affect water evaporation. They placed water basins beneath two different agri-PV configurations, with a third basin in an open reference area, and measured evaporation rates across all three setups over an extended period. One configuration featured an open design that allowed significant rainfall to reach the ground, while the other used semi-transparent modules covering the area.
You can read the results of the Chinese research project and the full report in our special on photovoltaics for farmers. (su)
