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A before-and-after impact study carried out on six French lakes over three years has revealed that photovoltaic coverage reduces water temperature by 1.2°C on average. The reduction in water temperature also occurred in areas of the lakes that were not covered with modules. French researchers have conducted a study on the impact of floating photovoltaic (FVP) plants on water temperature in freshwater ecosystems. They have used a before-after-control-impact (BACI) framework to understand these differences. The team measured six similar lakes, three with FVF and three without, over a three-year period. “Although the effects of FVF on water temperature are among the most frequently studied impacts of this emerging technology, most empirical research has focused primarily on small-scale pilot installations and short-term observations, greatly limiting the robustness of our knowledge,” they noted. «FVF plants are becoming a conspicuous part of the landscape and, to date, long-term ecosystem-scale empirical assessments of the global effects of FVF on lake temperature, with robust experimental designs, are still lacking.» This study was conducted between December 2020 and December 2023 in six lakes near gravel pits located in the Garonne River floodplain in southwestern France. Temperature measurements were made using buoys attached to a mooring with loggers deployed to a depth of four metres. Approximately one year after data collection, FVF was installed in three lakes, allowing a before-and-after analysis to be carried out. FVF plants covered 40.3%, 51.5%, and 55.5% of the lakes, with an average coverage of 49.1%. Each FVF-covered lake, or “impact” lake, was matched with a “control” lake without FVF. They were matched based on their hydromorphological similarities, as established by a principal component analysis (PCA) performed on environmental parameters. These environmental parameters were surface-to-volume ratio, maximum depth, perimeter, and elevation. According to the results, before the installation of FVF, the average water temperature was 15 ºC and 15.3 ºC in the lakes with FVF and without FVF, respectively; while after installation, the average was 16.9 ºC and 18.1 ºC. Overall, the presence of the FVF plant reduced water temperature by 1.2°C compared to lakes without the FVF plant, the academics noted. This temperature reduction varied significantly between seasons. In spring, an average difference of 2.3 ºC was recorded between the control and impact lakes, while in summer the average reduction was 1.9 ºC. The reduction was much smaller in winter and autumn, with an average difference of only 0.6 ºC. “On average, a water temperature reduction of 0.08 °C was observed between FVF and non-FVF lakes when air temperature increased by 1 °C. When the daily mean air temperature was <10 °C, the expected water temperature reduction was very limited (<0.5 °C). When the daily mean air temperature was >30 °C, the water temperature reduction was >2 °C. The largest differences in water temperature reached >3 °C,” they added. “In lakes containing FVF plants, the water temperature measured in the FVF-covered area did not differ significantly from the water temperature measured in the uncovered area.” To conclude their research, the team added that “in the context of climate warming, decreasing summer water temperatures could benefit freshwater organisms, but these benefits could be offset by other negative impacts, such as decreased dissolved oxygen and modifications in the carbon cycle (C cycle), including greenhouse gas emissions. Therefore, the cascading effects of FVF on freshwater biodiversity and ecosystem functioning remain to be assessed.” The results were presented in “ Floating photovoltaics strongly reduce water temperature: A whole-lake experiment,” published in the Journal of Environmental Management . Scientists from the French University of Toulouse and the Claude Bernard University Lyon 1 led the study. |
